Hello Codeforces!

I am in the process of making improvements and updates to the judgment machines. I've read the post https://codeforces.net/blog/entry/94587 and I think, maybe it is a good idea to make such a compilation line -O3 -funroll-loops -march=native -mtune=native? I haven't done any research that it is definitely better than -O2 and it is best in the general case for CP solutions. In a way, this will only strengthen the gap from Python/PyPy/Java, on the other hand: in pragmas and so you can set up everything. What do you think? What are suggestions to the command line?

P.S. You got it right. Yes, gcc11 std=c++20, pypy3 64-bit and more are coming.

Hello, Codeforces.

We want to run a series of load testing in the production environment for running contests. For this reason, Codeforces Round 739 will be temporarily extended (in order to test it at the current round). We will temporarily disable the ability to send solutions to it so that its extended duration will not affect the results. After the completion of the work, the duration of the round will be returned.

Our current plan is to finish today.

Mike

Hey!

As I don't see the post discussing the round, I decided to write this.

I do not participate much in contests now. But I love Code Jam. Thanks to problem writers and organizers!

It seemed to me that this round was no more difficult than the previous one. How do you like it?

I liked the problems:

• A (tags: interactive, constructive): strange problem — the most naive approach fits into the requirements on the total cost of requests;
• B (tags: number theory, dp): I wrote some kind of DP with memorization of $$$(a, s)$$$, where $$$a$$$ is the last element in the sequence we already placed (I build in increasing order) and $$$s$$$ is the total sum for the future elements. I don't have a proof why it works fast, probably because of the number of divisors is small.
• C (tags: combinatorics, recurrence): the last occurrence of 1 is the position of $$$n$$$, so we can divide sequence with this position on two parts and use DP to calculate the answer (don't forget to multiply on binomial coefficient);
• D (tags: graphs, flows, greedy): I used min-cost-flow to match Ms on the first field and the second in optimal way. I tried all possible values of flow to iterate over all possible pairs of Ms I want to match and took best cost among all choices.

Всем привет!

Сегодня мы открываем еще один курс в разделе EDU. Этот курс будет создаваться силами энтузиастов из сообщества. Первое занятие в нём подготовил Егор peltorator Горбачёв, тема — "Префиксные суммы и разностные массивы". Мы уже знакомы с его замечательными учебными видеоматериалами. Спасибо за помощь, Егор!

Типичное занятие раздела EDU содержит серию шагов, состоящих из видеороликов, конспекта и набора учебных задач.

Мы будем рады инициативе со стороны авторитетных членов сообщества по подготовке следующих занятий. Пока мы только начинаем, но надеемся на вашу помощь. Текущее требование к авторам занятий простое — красный цвет аккаунта сейчас или когда-либо в прошлом. По вопросам и предложениям надо писать Павлу pashka Маврину — координатору и завучу курса EDU от сообщества.

Hi Codeforces!

Yes, I know it's already April. Time flies so fast lately! Exactly a year ago we were just learning all the "delights" of lockdown.

So let's remember this 2020! What was it like? I don't know about you, but we had a hell of a lot of work.

I will now show you some graphs and you will be very surprised. Lockdown effect? Of course, but we did our best too.

Many thanks to everyone who did not stand aside! In 2020, we celebrated our 10th anniversary and crowdfunding money helps us grow and move forward. You helped us and each other. Thank you. Thank you for your contribution!

We've been working hard all year to make this community a better place. Did it work out? It's up to you to judge. Many thanks to everyone who invested time and effort:

• geranazavr555, kuviman, cannor147 helped a lot with development and new features;
• Una_Shem provided great organizational assistance;
• our problem coordinators are the greatest: KAN, antontrygubO_o, isaf27, 300iq, significant assistance was provided by BledDest, adedalic and budalnik, vovuh for preparing Div3 rounds;
• stingray advised and helped set up the network;
• our friends from VK for their help in our infrastructure;
• and help of problem writers and testers is immortal!

Time to take stock of 2020.

Partner Events

We are pleased to hold programming competitions with companies or for companies. I'm sure this is a great way to support the community of young programmers and hire talented candidates. Here is a list of our main partners this year:

• Telegram and personally Pavel Durov is supporting Codeforces activities for many years, every regular round is held with their help, thank you!
• ICPC — in 2020 we held several joint events (and we liked it!), I hope there will be more
• Mail.Ru — Technocup and Russian AI Cup
• VK, VK Cup — now a personal multi-track competition for Russian-speaking programmers
• XTX Markets, Codeforces Global Rounds — a series of 6 rounds with a separate scoring and souvenirs for the best participants
• Harbour.Space University — a series of educational rounds, the selection of summer school Tech Scouts
• JetBrains, Kotlin Heroes Rounds — Kotlin programming language contests
• Huawei — research competitions (marathons) with elements of machine learning
• Raiffeisenbank — round for both divisions with cool prizes
• Grakn Labs — round for both divisions with money prizes
• Microsoft, Microsoft Q # Coding Contest — unusual quantum computing competition

Hello, Codeforces!

I understand that 2021 has been going on for a long time, but here I have picked up a subset of some of the improvements that the Codeforces team made in 2020. Soon I will publish numbers (no, charts) with statistics for 2020. In the meantime, I bring to your attention a list of changes and improvements.

By the way, this is a decent list. This is about half to a quarter of all changes. It's just that other changes are more often somewhere in the internals of the system and are not visible to users. Please read this list. Each item is the effort of someone from the team. Thanks to geranazavr555, kuviman and cannor147 for their efforts. You've made our platform better! Well, by the way, I don't quit programming and many improvements were made by me.

The items on the list are written in a concise and informal form, many of the items I just copied from commit messages from git. If you want more details — ask in the comments, we will tell you! Of course, I forgot to include some improvements in the list.

Hello Codeforces!

This unusual and difficult 2020 has come to an end! What will the new year be like? I wish all of us that we have the opportunity to meet in person so that the contests take place not only online. I ask you to think of something important and good. Let this wish come true!

We have had a large number of rounds this year. I hope we have pleased you with interesting problems. Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2021!

I wish the participants victories over yourself (and over problems!) and development of your skills, interest and curiosity!

Hello, 2021!

And we have traditional gifts!

Change Handle Feature

Hurry! Only until the 10th of January, you can change your handle (but only once)! Note that it will be possible to roll back the changes or change the handle again only after a year. Be careful what you wish for.

You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.

Again, this year if you took part in at least 10 rounds you can request a handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 180 days, this period should be in 2017 or earlier. The inactive participant can't have posted comments, messages and so on. It can't take part in more than 2 contests. It will be automatically renamed and informed by email. A user has the opportunity to request back his/her handle: in this case, we will roll back the change and return your previous handle to you. If you can't change your handle to another, it means that some requirement doesn't meet. Please do not ask me to do something with it. Especially, I do not like requests like "this is my second account, I took an interesting handle myself". I'm not Santa Claus.

Talking about handles I always
reminisce the following story. Once a user wrote me the message: "Please change my handle from I_love_Valya to I_love_Sveta, as I no longer love Valya ..."

New Year's Masquerade of Colors and Ranks

The traditional magical tab has appeared in the profile setting. Happy New Year!

— MikeMirzayanov

You can view PDF version of the tutorial by the link.

Добрый день!

15-го ноября состоялся Четвертьфинал Южного подрегиона NEERC (Northern Eurasia) чемпионата ICPC. В этом году соревнование проходило онлайн. Соревновались 108 команд, многие из которых получили приглашение по результатам квалификационного этапа.

Совсем скоро в пятницу, 25-го декабря в 14:35 (МСК) состоится онлайн-зеркало 2020-2021 ICPC, NERC, Южный четвертьфинал (онлайн-трансляция, правила ICPC).

Надеюсь, вам понравятся задачи. Председателем жюри этого соревнования являюсь я, а над задачами работал дружный коллектив жюри экс-участников чемпионата из Саратовского ГУ и иногородние члены жюри: adedalic, ashmelev, BledDest, DmitryKlenov, DStepanenko, elena, KAN, kuviman, MikeMirzayanov, pashka, PavelKunyavskiy, Дмитрий Мещеряков, Герман Наркайтис. Спасибо!

Отдельные лучи благодарности тестерам: Merkurev, Um_nik, romanasa, josdas, budalnik, Perforator, Oleg_Smirnov, IlyaLos, Supermagzzz, Stepavly, Igor_Kudryashov, HolkinPV, Edvard, le.mur!

Приглашаю команды ICPC к участию и просто индивидуальных участников соревнований Codeforces принять участие!

Конечно, соревнование будет нерейтинговое. Участников официального соревнования я прошу воздержаться от участия в зеркале и обсуждении задач до его окончания.

Так как в этом году аудитория участников была шире (из-за онлайн-формата), то и задачи мы подобрали чуть доступнее, чем в прошлые годы. Если вы из топ-команды, которая претендует на медаль в Финале ICPC, то, возможно, веселее вам будет решать этот контест лично.

Удачи!

In 2020, with the support of XTX Markets, 6 rounds of the Codeforces Global Rounds were held. Each round was common for both divisions. All such rounds were rated for all participants. At each such round, 50 brand T-shirts were handed out, and we gave T-shirts to all problem writers!

I say a big thank you to XTX Markets (and personally to Yuri Bedny and Alexander Gerko) for supporting Global Rounds. Without your help, we could not host them! I sincerely admire companies like these who come to support and develop the community that some of their employees come from. It will be great if other companies follow the example and also offer their help. We need it!

The prizes for the 6-round series in 2020:

• In each round, top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Final results (and congratulations!):

Mail.ru Group совместно с МФТИ, МГТУ им. Н. Э. Баумана и Codeforces в шестой раз запускает «Технокубок» — олимпиаду по программированию для школьников. В этом году «Технокубок» вошел в перечень олимпиад первого уровня.

Сразиться за звание самого талантливого молодого программиста приглашаются учащиеся средних образовательных учреждений 8-11 классов.

Победители и призеры олимпиады будут определены по результатам очного этапа, который будет проведен 7 марта 2021 года на базе МФТИ, МГТУ им. Н.Э.Баумана, а также на других региональных площадках, о которых будет сообщено позднее.

Привет, Codeforces!

Есть кое-что новое. Кажется, теперь переиспользовать задачи с раундов в учебных и прочих целях станет проще.

Как вы знаете, в мэшапы можно добавлять задачи с прошедших раундов Codeforces просто по их кодам вида 1234D (айдишник контеста + буква задачи). Я сам регулярно этим пользовался, когда преподавал в Саратовском университете — очень удобно было так делать тренировки: задачи ожидаемой сложности и тематики, есть русское и английское условия, можно посмотреть разбор и множество решений. Почти всюду очень сильные тесты по причине поддержки взломов и довзламывания, задачи хорошо подготовлены. Кроме того, с помощью списков пользователей можно для группы учащихся легко подобрать задачи, которые они не решали.

Но была и проблема: задачи в таких тренировках легко гуглились. И если взрослые участники в большинстве своем могли справится с искушением подсмотреть решение или тесты, то новички иногда поступали нечестно. Я получал несколько просьб придумать что-то с этим недостатком. Одна из последних таких просьб была от Daniel Darooha Sleator (https://en.wikipedia.org/wiki/Daniel_Sleator). Спасибо за идею!

Итак, кажется, работа завершена. Встречайте новую возможность на Codeforces: теперь вы можете переписать условия для задач в мэшапах, полностью заменив его на своё собственное. Подробнее о возможностях нового функционала:

• в мэшапах для задач появляется новый ссылка-значок , перейдя по которому будет форма для создания/редактирования нового условия;
• вы можете использовать оригинальный текст условия как шаблон при написании собственного: будьте очень аккуратны — вы должны в точности повторить все детали условия, чтобы задачи формально не различались;
• эта форма сравнивает набор формул в оригинальном тексте и новом условии и если они различаются как множества, то будет соответствующее подробное предупреждение;
• вы можете установить собственные тесты из условия (примеры) — здесь тоже надо быть очень внимательным, чтобы они были формально корректными;
• если вы добавили примеры, то сначала решения будут протестированы на них, а лишь затем на официальных тестах;
• вы можете загрузить собственные файлы с изображениями и использовать их в условиях;
• есть настройка, чтобы временно выключать использование нового условия (например, если оно еще в разработке).

Учитывая, что в архиве сейчас уже более 6500 задач, открывается отличная перспектива для организации тренировок и учебных занятий.

Надеюсь, что нововведение будет полезно. Пишите в комментариях, что можно улучшить и/или исправить.

М.

During the round Grakn Forces 2020 many of you can notice that now we highlight diff in statements if they changed. Do you like this new feature?

Also, I'm thinking on the way to automatically notify participants if statements changed. I still do not fully understand how this should ideally be implemented. Sometimes it is true that just a forgotten comma was added. On the other hand, it is almost always better to notify of changes. What do you think?

In 2020 (as in 2019), with the support of XTX Markets, 6 rounds of the new Codeforces Global Rounds will be held. These will be common rounds for both divisions of 7–9 problems each. The duration of the rounds will be 2-3 hours, depending on the number and complexity of the problems. All such rounds will be rated for all participants. At each such round, 50 brand T-shirts will be handed out, and we will be happy to give T-shirts to all problem authors and problem testers.

The prizes for the 6-round series in 2020:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Current standings after four rounds (sorted by total points in four rounds):

Привет!

Новым занятием в разделе EDU мы начинаем цикл, посвященный графовым алгоритмам. Это занятие ориентировано на самых начинающих — в нём изложены основные определения и понятия теории графов.

Общий список занятий в "ITMO Academy: пилотный курс" теперь выглядит так:

• z-функция
• суффиксный массив
• дерево отрезков, часть 1
• дерево отрезков, часть 2
• двоичный поиск
• система непересекающихся множеств
• введение в теорию графов

Подробнее об учебном подразделе на Codeforces (и его β-тестировании) можно прочитать по ссылке.

Большое спасибо le.mur за монтаж видео, а также студентам ИТМО Supermagzzz и Stepavly за подготовку практических задач.

Занятие состоит из 4 шагов:

• неориентированные графы (7 задач);
• классы графов (4 задачи);
• ориентированные графы (4 задачи);
• представления графов (4 задачи);

Надеюсь, что получилось интересно и полезно.

Всем привет!

Это просто небольшой анонс последних обновлений в разделе EDU. Появилось такое:

• Теперь вы можете просматривать чужие решения, если сами решили задачу. Мне кажется это может быть очень полезно. Когда я был участником соревнований, то всегда смотрел как ребята поопытнее меня написали тот или иной код. Частенько находил что-то интересное и полезное.

• Теперь вы можете просматривать тесты к задачам в формате, как это сделано в архиве задач основного раздела сайта.

• Теперь вы можете довзламывать чужие решения, если сами решили задачу. И весело и позволит улучшить тестовую базу этих задач.

Подробнее о разделе EDU можно прочитать по ссылке.

А какие есть пожелания к функциональности у вас? Про фильтр результатов практики по друзьям помню, сделаю.

UPD: А еще для всех контестов практики я установил политику "пересчёта ограничений по времени". Это недавнее нововведения — вы можете это использовать для учебных контестов, система будет автоматически увеличивать ограничения по времени для некоторых языков. Рекомендуется к использованию только в учебных соревнованиях.

Hello.

Last days I did many improvements in EDU. I afraid it can affect the contest interface. So I invite you to take part in [contest:1390], just to test the system. Please, try to hacks: there were some changes in them. Please, don't expect new or interesting problems. It is just a test. Unrated.

The problems contain extremely weak pretests. Time limits are too tight. I made my best to increase number of hacks :-)

Thanks, Mike.

UPD: Thanks! It seems no bugs have been found.

Hello.

Unfortunately, the Educational Codeforces Round 91 round will be unrated and, actually, it was completely ruined. I don’t understand what happened yet. At some point in time, all systems began to work unstably and actually stopped working. So far I have no understanding what happened. You may have to change the schedule for future rounds. Sorry. We have such a black stripe. Very upset and demotivated.

Mike.

In 2020 (as in 2019), with the support of XTX Markets, 6 rounds of the new Codeforces Global Rounds will be held. These will be common rounds for both divisions of 7–9 problems each. The duration of the rounds will be 2-3 hours, depending on the number and complexity of the problems. All such rounds will be rated for all participants. At each such round, 50 brand T-shirts will be handed out, and we will be happy to give T-shirts to all problem authors and problem testers.

The prizes for the 6-round series in 2020:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

Current standings after three rounds (sorted by total points in three rounds):

Hello Codeforces!

In February, in the Russian-language interface, we announced a new educational subsection of the website. It's time to go international!

I am pleased to invite you to test the new section, which so far has the working title EDU. I hope that someday EDU will become a real educational hub for fans of programming competitions. It will be great to have in one place the collected educational content with text and video materials, with selected thematic problems.

At the moment, its functionality is still under development, and here is the current progress and β-testing preview. Write your comments and suggestions in the comments here.

So far, only a pilot course with the first lesson "Suffix Array" has been presented to your attention. The plans include expanding both the functionality of the section and filling it with new content.

Russian version already contains more lessons. At the moment, it is important for us to get the first feedback from international users. Many thanks to Pavel Mavrin (pashka) for preparing the first English lesson. Please support pashka to motivate him to translate more lessons!

I hope that in the future, using the materials in this section, you will be able to gain knowledge on an extensive set of topics that are used in contests and olympiads. According to the current plan, a group of ITMO lecturers and students will be engaged in a pilot course, a little later we will consider the possibility of opening the possibility of creating classes for a wider audience.

Thanks to ITMO University for the idea and support for the implementation of this initiative!

Hello!

Together with colleagues Schaffhausen Institute of Technology we hold such an event. If you want to study in Switzerland, then perhaps this is your chance!

— Mike

Hello, Codeforces!

We are thrilled to announce a new SIT STAR Contest by the Schaffhausen Institute of Technology in Switzerland. The winners will have a chance to get a fully-funded Master’s scholarship in Computer Science and Software Engineering.

What is the SIT STAR Contest?

The goal of the SIT STAR Contest is to promote interest in the field of Computer Science and Software Engineering, give students an opportunity to demonstrate their knowledge of programming, and be considered for a fully-funded graduate scholarship. You can apply to the contest here!

The SIT STAR Contest consists of:

1. June 1-7, 2020 | Practice: To familiarize yourself with the testing environment, you will first be granted access to a practice round. You can practice any time from 1st to 7th June 2020. This is an optional step but we highly recommend to take part in it. The results of this round won’t affect the final score.
2. June 17, 2020 | SIT STAR Contest: The final round will take place on 17th June. The participants will be given 4 hours to complete it.
3. June — July, 2020 | Interviews and Winners announcement: Top participants with the highest scores will be invited for the interviews with the professors from the Schaffhausen Institute of Technology.

The SIT STAR Contest will include 8-12 problems of various levels of difficulty in algorithmic programming.

Hello, Codeforces!

I carefully read most of the ideas that are written in the comments and posts. I have no way to answer everything. Otherwise, I would only do what I answered. Excuse me. There are many good ideas, some easier to implement, some more difficult.

The following idea seems vivid to me. Idea is to limit the maximum rating increase for rounds where there is already an upper rating limit at registration. For example, if the upper limit is $$$u$$$, then your rating after participating in the round cannot be higher than $$$u+100$$$ even with a very successful performance. For example, after winning in Div.3 round the rating can't be more than $$$1699$$$.

It is easy to implement, it does not entail a significant redistribution of ratings. It seems I like it. What do you think about it?

Actually, probably a good idea is to make a bottom cap for rating changes: in any case of any (even the worst) performance, the rating shall not fall by more than $$$100$$$. Еverything happens during the rounds — there may be technical difficulties, force majeure, or just catastrophic bad luck. What are your thoughts about it?

— Mike

Всем привет.

В ближайшие дни я закончу реализацию небольших изменений подсчета рейтинга для новых аккаунтов. Вот основные нововведения.

• Рейтинг нового аккаунта будет равен $$$0$$$ для отображения (но будет равен $$$1400$$$ при подсчете изменений рейтинга).
• Допустим, после первого раунда участник на основании своих результатов получил $$$d_1$$$ единиц рейтинга (помните, что его рейтинг считался равным $$$1400$$$ при таких расчетах), то в отображении рейтинга после этого раунда будет отображен $$$500+d_1$$$. Таким образом, после первого участия рейтинг наверняка увеличится с $$$0$$$ до значения в районе $$$500$$$ (плюс-минус $$$300$$$ примерно).
• Таким образом, перед вторым участием отображаемый рейтинг равен $$$500+d_1$$$, а рейтинг для подсчета изменений $$$1400+d_1$$$. Допустим, новое изменение будет равно $$$d_2$$$, тогда отображаемый рейтинг станет равен $$$500+d_1+350+d_2$$$.
• Таким образом, перед третьим участием отображаемый рейтинг равен $$$500+d_1+350+d_2$$$, а рейтинг для подсчета изменений $$$1400+d_1+d_2$$$. Допустим, новое изменение будет равно $$$d_3$$$, тогда отображаемый рейтинг станет равен $$$500+d_1+350+d_2+250+d_3$$$. И так далее первые $$$6$$$ раундов.
• Поощрения отображаемого рейтинга будут равны $$$500, 350, 250, 150, 100, 50$$$ (в сумме ровно $$$1400$$$). Следовательно, после участия в $$$6$$$ раундах рейтинг «стабилизируется» и отображаемый рейтинг будет равен тому, который используется для пересчета изменений.

В среднем рейтинг нового аккаунта первые раунды будет возрастать, приближаясь к настоящему значению. Эти нововведения помогут решить несколько проблем:

• Мы уменьшаем стартовый рейтинг с $$$1500$$$ до $$$1400$$$, чтобы новые аккаунты вносили меньший вклад в общий рейтинг.
• В настоящее время особенно для неопытных участников есть эффект, что их рейтинг в начале участия постоянно падает (от $$$1500$$$ до того значения, которое является верной оценкой их умений). Это, конечно, приводит к дискомфорту – ты стараешься, участвуешь, что-то решаешь, а рейтинг падает.
• Предыдущий пункт дает дополнительную мотивацию к созданию новых аккаунтов. Если твой текущий рейтинг менее 1500, то есть искушение начать с чистого листа со свежим аккаунтом. После изменений такой старт потребует заметных длительных усилий, чтобы вскарабкаться хотя бы твоего текущего рейтинга. Кажется, что изменения должны слегка уменьшить мотивацию создавать новые аккаунты.

Видимо, подобная идея используется и на AtCoder, но подробностей я не знаю.

Как вам такое нововведение?

I almost copied my previous blog post to separate two discussions: about Div. 4 in general and about Round 640. Please, visit this blog post to discuss Div. 4 rounds.

Hello Codeforces!

Yes, it is not a mistake I decided to run the first Div. 4 round: Codeforces Round 640 (Div. 4)! It starts on May/09/2020 17:35 (Moscow time).

The format of the event will be identical to Div. 3 rounds:

• 5-8 tasks;
• ICPC rules with a penalty of 10 minutes for an incorrect submission;
• 12-hour phase of open hacks after the end of the round (hacks do not give additional points)
• after the end of the open hacking phase, all solutions will be tested on the updated set of tests and the ratings recalculated
• by default, only "trusted" participants are shown in the results table (but the rating will be recalculated for all with initial ratings less than 1400 or you are unrated participant/newcomer).

I urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism, but simply to take part unofficially. Please do not spoil the contest to the official participants.

Only trusted participants of the fourth division will be included in the official standings table. This is a forced measure for combating with unsporting behavior. To qualify as a trusted participants of the fourth division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1600 or higher in the rating.

Regardless of whether you are a trusted participant of the fourth division or not, if your rating is less than 1400 (or you are a newcomer/unrated), then the round will be rated for you.

Please, read some of my thoughts about the problems of this round:

• Don't expect A+B level problems :-) It is a real contest with easy but problems. I hope most problems fit in range D3A-D3D problems.
• I'm not sure about order of problems. Try to read all problems or at least most of them. Testers expressed skepticism about the order of the problems but suggested different pairs of problems to swap.

We are just starting, so the complexity of the problems may deviate somewhere from your or my expectations. Wait and see.

The first "thank you" is sent to antontrygubO_o, who helped with the review and selection of problems.

Many thanks to the testers: pashka, 300iq, vovuh, Golovanov399, spookywooky, kocko, Sho, igor_kz, Sho, scott_wu, bfs.07.

I hope you enjoy the problems, and the round will be interesting!

Please, leave in comments your thoughts about exactly this round. If you want to write something about the general idea of Div. 4, then visit this blog post.

UPD: Editorial is published!

Hello Codeforces!

Initially, here was an announcement of the first round for the fourth division. But currently, in comments there is the huge discussion of division 4 rounds in general. Honestly, I read all (almost all) comments.

I decided to replace the text of this post to match the comments better. Also, I'll write one more post separately to discuss problems. And this post will be about division 4 rounds in general.

About Div4 rounds:

• I'm not afraid of a queue or servers overload: it is technical challenges and my task is to resolve them. The previous biggest rounds were not overshadowed by technical problems and judging delays, they went well. I see no reasons why something should work much worse in the rounds for Div. 4 (and if it does, then this should be fixed).

• I don't think that Div. 3 rounds are too dificult. Actually, they are quite good for beginners. They are noticeably easier than Div. 2. Almost every round trusted participants solve all problems, many participants solve all without one problem. I think in general they are OK.

• Now I don't like the idea to run in parallel Div. 3 and Div. 4 rounds (like we do for Div. 1 and Div. 2). Div. 3 rounds are harder to prepare and I don't think we can host them more than once per ~2 weeks. It means that such rounds will not give new contests for newcomers and grays/greens (I think Div. 3 are already interesting rounds for them to take part). But separate Div. 4 rounds will be easier to prepare (even than Div. 3) and probably making them we can offer more contests for low rated participants.

• I'd like to repeat: Div. 3 and Div. 4 rounds doesn't affect the rate of Div. 1 and Div. 2 rounds. The coordinators focus only on Div. 1 and Div. 2 rounds. All Div3 (and future, if any, Div4) rounds are prepared without any help from the coordinators.

• I do not think that by hosting Div. 4 rounds, we will lower the quality of problems. Now, I don't plan to use absolutely A+B level problems even for the Div. 4. I think problems of level like Div3A-Div3D are good candidates.

Thanks for your attention to this idea!