Hello Codeforces!

omeganot and I are excited to invite you to participate in Codeforces Round 919 (Div. 2) which will start on Jan/13/2024 17:35 (Moscow time). You will be given 2 hours to solve 6 problems. One of the problems is divided into 2 subtasks.

This round will be rated for participants of Division 2 with a rating lower than 2100.

We would like to thank

• Artyom123 for his epic coordination and also for translating the problem statements!
• A_G, PurpleCrayon, Yam, Andreasyan, oursaco, hyforces, Apple_Method, GusterGoose27, null_awe, awesomeguy856, lunchbox, jli505, Lemur95, ocasu, EmeraldBlock, EnDeRBeaT, cry, htetgm, SriniV, nonrice, mark, thehunterjames, ETL, AndrewPav, ForgottenDestiny, lazipotato, dutin, and Saq56 for testing the contest and providing valuable feedback.
• MikeMirzayanov for the amazing Codeforces and Polygon platforms.
• You for participating!

The score distribution is $$$500 - 1000 - 1500 - 2000 - 2250 - (2500 - 2500)$$$.

We hope everyone will enjoy the round!

UPD 1: Editorial

UPD 2: Congratulations to our winners and first solves!

Hi everyone!

Sponsored by

After successful camps last year (winter wrap, fall wrap), we are happy to announce that Osijek camp will be returning on 17.-25. February 2024 — just in time to prepare for the World Finals in... Maybe this spring? As well as several regional contests throughout the fall. The camp is hosted by the School of Applied Mathematics and Computer Science of the University of Osijek and is brought to you by the organizing team of adamant and -is-this-fft-.

The camp is inspired by various competitive programming camps that we attended during our active years in ICPC, and is aimed to help college students prepare for ICPC regional contests and finals. The camp will consist of 7 ICPC-style contests and 2 days off.

If you want to get a feel of the contests, two contests from the last edition will be featured in Universal Cup contests soon:

1. Estonia contest, scheduled on 2024.01.20.
2. Delft contest, scheduled on 2024.02.03.

After Universal Cup rounds, the contests will also be uploaded to Codeforces gym.

Details

Participation fee for onsite participants is 150€ per person. It does not include regular meals, travel or accommodation. Some further details about location, travel and food options can be found on the website.

If you want to participate, but are unable to come onsite, we offer a reduced fee of 100€ per person for online participation. It is also possible to reduce fees individually if you are unable to attend some of the contests. This will be handled on a case-by-case basis.

We support and empathize with those affected by the ongoing war in Ukraine, therefore we offer a 100€ discount for affected individuals and teams affiliated with Ukrainian institutions. In other words, the fees would be 50€ and 0€ per person for onsite and online participation correspondingly.

The expected starting time for the contests is 10am CET. For online participants, it is still a preferred starting time, but we will make accommodations for other starting times.

Most of our contests are fresh and developed for this camp. A small number of contests may be based on previous contests that have not been released to the general public. If you have seen some problems of a contest before, you can't participate on that day (and your participation fee will be reduced accordingly). We will privately contact participants who might be affected. Based on feedback, we will have a silence period until the end of World Finals 2023, during which camp materials will not be released to the public. Therefore, we ask participants to not discuss the problems in public until that date.

Participants

If you are interested in participating, please fill the form here.

We ask you to register before February 9 if you want to participate online and before February 3 if you want to participate onsite.

Also, if your university or organization has a lively ICPC community that may be interested in attending the camp, and you have some contacts of people in charge (e.g. coaches) we would highly appreciate if you could fill the form here, so that we can send an invitation. Thanks!

Problemsetters

We'd like to thank and praise the authors of the contests in the camp:

• qwerty787788 — ICPC 2015 World Champion, Google HashCode 2019 and 2020 winner, CodeChef SnackDown 2016 and 2019 winner.
• antontrygubO_o — author of 1188B - Count Pairs.
• jeroenodb — Codeforces International Grandmaster, Computational geometry enjoyer. NWERC 2023 silver medalist.
• 998kover — IOI gold medalist, problemsetter for IZhO and data structure enjoyer.
• TheScrasse — Codeforces International Grandmaster, SWERC 2023 silver medalist, Codeforces coordinator.
• KLPP — Codeforces International Grandmaster, SWERC 2023 silver medalist, IOI silver medalist.
• Lebossle — Codeforces Grandmaster, problemsetter for ICPC Latin America 2021+.
• adamant — Codeforces Grandmaster, maintainer of cp-algorithms.com, Osijek camp organizer, author of over 60 competitive programming problems.
• gen — ICPC 2012 and 2014 world finalist, Google Hashcode 2017 finalist, coauthor of 7 Codeforces contests.
• de_sousa — SWERC 2023 silver medalist.
• flamestorm — MIT student, author of over 50 competitive programming problems.

... And others. We would also like to thank Um_nik and errorgorn for their help with reviewing problem proposals.

You can find more details about contest rules and technical setup on the website.

Sponsors

Last but not least, we would like to say special thanks to our sponsors, who make the camp possible. If you are interested in sponsoring next editions of the camp or have any questions, please feel free to reach out at ocpc (at) mathos (dot) hr.

We would also like to thank Codeforces for guidance and promoting this announcement to the main page, eolymp for providing us an online judge for the contests and the School of Applied Mathematics and Computer Science of the University of Osijek for all their organizational support and providing us a physical location to conduct the camp.

Finally, we kindly thank ICPC foundation for their help and support.

Hello, Codeforces!

For the new members of the community, I would like to remind you that Polygon (https://polygon.codeforces.com/) is a system developed and maintained by Codeforces for preparing programming problems. It is there that authors and coordinators develop all the problems for the rounds. Moreover, I believe a significant (large?) portion of the problems for other competitions is also developed there: various stages of ICPC, national competitions of different levels, educational problems for various courses, etc. In 2023, more than 50000 problems were prepared in Polygon (only those for which a package was compiled are counted)!

Large language models are becoming an increasingly integral part of our lives, taking over routine and monotonous tasks.

With their help, problems created in Polygon will now have fewer typos and incorrect language constructions.

We have recently implemented three features in Polygon to ease routine tasks:

• Translation of solutions into another programming language
• Translation of problem statements into another language
• Advice on improving the texts of problem statements

All these features operate on request and typically take 30-120 seconds for a response from the AI service (currently, only OpenAI GPT in our case).

While waiting for translations of solutions and statements is acceptable, waiting for advice on improving text was too tiresome.

I recently developed several problems and, despite my experience in writing statements (and even having the highly recommended Grammarly plugin in my browser), the automatic advice tool for improving text helped enhance them.

Another example: I checked the working versions of the Hello 2024 problems about a day before the round started and took the time to manually launch the edit prompter. In most (almost all?) problems, it indeed suggested errors or deficiencies in English/Russian. The corrections were minor, but why leave mistakes if they can be fixed?

Indeed, such a tool can greatly improve the grammar of statements, but using it on demand was tiresome.

As a result, I implemented background processing of the problem statements to simplify and speed up the application of this feature. Now, Polygon analyzes problem statements in the background to find shortcomings. If any are found, you will see a similar block in the problem's sidebar.

This block might not appear immediately upon changing the statement, as processing a request to the AI service usually takes tens of seconds.

Clicking on the review link will open a tool for merging changes.

This is a synthetic example. I made a few typos in the statement of my old problem. I should note that even the Grammarly plugin didn't catch all of them.

I circled one of the triangles. Click on them to transfer the proposed changes (on the right) to the text of the statement (on the left). You can only transfer for a whole paragraph at a time (that's how the tool works), but you can manually correct the left if you want to transfer part of the changes or modify them.

At the bottom of the form is a Save button that will save the updated statement.

I hope that authors and coordinators will not neglect this feature and that there will now be fewer typos and language errors in statements.

I would like to note that I see no reason to worry about data leakage when processing statements with the AI service. For example, OpenAI explicitly states that data from API requests cannot be used for training: https://help.openai.com/en/articles/5722486-how-your-data-is-used-to-improve-model-performance. After all, we've been googling problem ideas for many years to ensure the problem is new. It seems to be a similar case.

However, if you are still concerned and want to explicitly disable this feature, it can be done:

• For a problem: via the Advanced link on the General info tab.
• In user settings: disable this feature for all new problems you create.

Thanks for reading! Stay tuned for more updates.

Hello.

Today I removed the account zh0ukangyang from the rating, after first nullifying its results in the Pinely Round 3 (Div. 1 + Div. 2) and banning it.

I would like to remind you again: Codeforces insists on the policy of using a single account. Creating and using additional accounts violates this rule. In an ideal world, each Codeforces account corresponds to one specific person.

Sometimes additional accounts are created for official, usually collective purposes (writing a post on behalf of a company, etc.). In such cases, these accounts do not participate in rated rounds and we do not pay attention to them unless there is some abuse of Codeforces resources.

Unfortunately, it is difficult for us to enforce this rule in almost all cases. But I urge you to respect the rules. Please remember that by registering for any rated contest, you confirm that you are participating with your only account. If this is not the case, then you are violating the community rules. Do not do this. By breaking the rules, you automatically agree that punishment may be applied to you.

In cases where rule violations significantly affect the ratings of other participants, we may take drastic measures. Remember, with great power comes great responsibility.

In this case, zh0ukangyang was at the top of the ratings, occupying high places in the rounds.

What should you do if you already have several accounts and want to stop breaking the rules? You should not write to us with a request to delete accounts or transfer data from one account to another. All you need to do is simply stop participating from all accounts except one. Change the password to a random one and do not log into the account anymore.

For participants at the very top (somewhere in the top 50 of the ratings), we are ready to manually deal with the situation and exclude illegal accounts from the rating.

M.

Hello Codeforces,

We are very glad to invite you to participate in Hello 2024, which will start on Jan/06/2024 17:35 (Moscow time). You will be given 8 problems and 2.5 hours to solve them. One of the problems will be divided into two subtasks. The round will be rated for everyone. There will be at most 2024 interactive problems, so please read the guide for interactive problems before the contest.

All the problems are written and prepared by me.

We would like to give our sincere thanks to:

• errorgorn for his wonderful coordination!
• Alexdat2000 for translating problem statements.
• dario2994 for coming up with the solution to one of the problems.
• conqueror_of_tourist, iLoveIOI, Um_nik, oolimry, thenymphsofdelphi, Kaitokid, Brovko, Scintilla06, zengminghao, MarcosK, lanhf, beepbeepsheep, DylanSmith, kymmykym, CoDeRoK, wery0, kai824, teruel, asiad, priyanshu.p, chromate00, Blagoj, tibinyte, htetgm, Guevara74, Amrharb, Joshi503, BuzzyBeez, 18o3, nor, Kuroni for testing the round.
• dantoh, Myrcella, dario2994, dreamoon_love_AA, jamessngg, pavement, bensonlzl for testing a subset of the problems in 2022.
• MikeMirzayanov for the great codeforces and polygon platform.
• You for participating in the round.

The score distribution is $$$250 - 500 - 1000 - 1500 - 2250 - (1500 + 1500) - 4000 - 5000$$$.

Hope everyone will enjoy the round!

Congratulations to the winners!

1. ecnerwala
2. ksun48
3. VivaciousAubergine
4. gamegame
5. cnnfls_csy
6. maroonrk
7. tourist
8. Geothermal
9. kmjp
10. yosupo

Congratulations to the first solves as well!

• A: Spawnpoint
• B: tourist
• C: tourist
• D: tourist
• E: tourist
• F1: ko_osaga
• F2: ko_osaga
• G: VivaciousAubergine
• H: rainboy (after the contest)

UPD: Editorial

Привет, Codeforces!

Я рад пригласить вас на Good Bye 2023, который пройдет в 30.12.2023 17:50 (Московское время). Он будет рейтинговым для всех участников.

Задачи были придуманы и подготовлены 74TrAkToR, zwezdinv, OR_LOVe, marzipan, platelet.

Мы хотим поблагодарить всех, кто оказал бесценную помощь в подготовке этого раунда:

• 74TrAkToR за блестящую координацию и полезные советы.
• Тестеров operasfantom, i_love_penguins, IzhtskiyTimofey, __Foam, a_ok, bibikov1, Kihihihi, ace5, kostylevGO, Pavarishko, Alex239, alex.kudryashov, htetgm, katzi, dasha..zhilina, green_gold_dog, RomkaRS, Mangooste, _IVON_, EJIC_B_KEDAX, maks_matiupatenko, plagues, platelet, Golovanov399, Endagorion, KbltQaQ за качественное тестирование раунда и важные советы.
• errorgorn, azureglow за идею и помощь с подготовкой одной из задач.
• MikeMirzayanov за прекрасные системы Codeforces и Polygon.

На раунде вам нужно будет решить 8 задач. У вас будет 2 часа на их решение.

Разбалловка: 250—750—1250—1500—2000—2750—3750—(2750+1750)

Желаем всем удачи и высокого рейтинга!!!

UPD: https://codeforces.net/blog/entry/124060

UPD1:Разбор опубликован

Happy Holidays Codeforces! 🎅

mesanu, flamestorm and I are very excited to invite you to Codeforces Round 918 (Div. 4)! It starts on Dec/28/2023 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 an unrated participant/newcomer).

We urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism but to take part unofficially. Please do not spoil the contest for 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 unsporting behavior. To qualify as a trusted participant of the fourth division, you must:

• take part in at least five rated rounds (and solve at least one problem in each of them),
• do not have a point of 1400 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.

Special thanks to the VIP testers: AlperenT, KrowSavcik!

Thanks a lot to the testers: Qualified, Kaushal_26, htetgm, MADE_IN_HEAVEN, sandry24, hbarp, Vladosiya, LucaLucaM, Gheal, tvladm, Dominater069, haochenkang, xiaowuc1, pashka, vrintle, BucketPotato!

And many thanks to Vladosiya for translating the statements!

We suggest reading all of the problems and hope you will find them interesting!

🎄 🎄 🎄 Good luck to everyone and enjoy the holidays!!! 🎄 🎄 🎄

Hi, Codeforces!

I'm glad to invite you to take part in Codeforces Round 917, which will take place on 24.12.2023 17:35 (Московское время). Round will be rated for participants with rating less than $$$2100$$$. Participants from the first division can take part out of competition.

There will be $$$6$$$ problems for $$$2$$$ hours. The problems are authored by IgorI, zidder and me.

Part of the problems in this round were in the Yerevan SU 28.1₀.₂₀₂3 Contest. If you participated in it or know at least one problem from it, please refrain from participating in this round.

We would like to thank

• Thanks to TheScrasse for excellent round coordination;
• Thanks to our testers: KAN, S_Voloch, Tmitmi, Pagode_Paiva, hgglmao, IceWolf898, daud04, dewana310, menborong, -dub-otrezkov-, rtckyw_, Bebo, Starswap, MCuong, line, shamir0xe, Adrisos, AbduSaber, pwned, TamimEhsan, Andreasyan, priyanshu.p, kzyKT, Lyde, mcdx9524, mihneacazan, PurpleCrayon, errorgorn, fishy15, maomao90, okwedook, higz, Akash., Kapt for giving valuable feedback for problems;
• Thanks to MikeMirzayanov for creating Codeforces and Polygon.

Scoring distribution: $$$500 - 1000 - 1500 - 2250 - 2500 - 3000$$$

We hope you'll like the problemset! Good luck and have fun!

UPD 1: Editorial

UPD 2: Congrats to our winners:

• Div1 + Div2

1. tourist
2. Sugar_fan
3. BurnedChicken
4. Rubikun
5. kotatsugame

• Div2

1. -Misaka-Mikoto-
2. _chashuibiao_
3. needy_and_sorry
4. Godjob
5. LordVoIdebug

Ciao, Codeforces! We're glad to invite you to take part in Pinely Round 3 (Div. 1 + Div. 2), which will start on Dec/23/2023 17:35 (Moscow time). You will be given 9 problems and 3 hours to solve them. One of the problems will be divided into two subtasks.

The problems were authored and prepared by me.

We would like to thank

• errorgorn for his fun 🤌 🤡 🤓 😩 coordination;
• Alexdat2000 for Russian translation;
• franv for writing $$$173$$$ lines of checker of problem H correctly at the first try;
• franv for pictures in the editorial;
• dario2994, Endagorion, ffao, Golovanov399, nor for VIP testing;
• Kaey, MrBrionix, lorenzoferrari, nicksms, fwitt, milind0110, jamesbamber, alls, dorijanlendvaj, sunsh1ne, BrehamPie, 31ballons, gamegame, AndreySergunin, zeliboba, Jarnsida, I_L0Ve_MySelF, 1L1YA, MrAndria, TheRubbish, omsincoconut, jerefigo, dazlersan1, t0rtik, nondeterministic, SetBackIsBest, vantaablackk, htetgm, BestCrazyNoob, LaMatematica14, OgradL, bitset, magnus.hegdahl, Mirali777, Nika533, silxi, ttamx, ApraCadabra, pera2008, Sokol080808, A_G for testing;
• MikeMirzayanov for creating Codeforces and Polygon.

Score distribution: $$$500 - 1000 - 1500 - 2000 - 2500 - (1500 + 1500) - 4000 - 6000 - 6000$$$

We hope you'll like the problemset!

Update 1: the editorial is here.

Update 2: congratulations to the winners!

This round is made possible with the support of Pinely!

Pinely is an algorithmic trading firm, with its main focus set on high-frequency and ultra-low-latency trading. They have offices in Amsterdam, Limassol, Singapore, and Shanghai and are open for job discussions. Pinely is a team of winners, awardees, and medalists of various competitions in respective fields such as ICPC, IMC, HITB PRO CTF, and Google HashCode, etc. They constantly face various challenges such as developing strategies for trading, optimizing trading systems to achieve the lowest latency reactions to various market events, and saving and processing large volumes of historical data.

You can find out more about Pinely on their website or from their employees registered here on Codeforces. If you want to join the Pinely team, please send your CV to [email protected] or fill in the form:

Prizes: top 30 contestants and 10 random contestants placed 31-100 will receive a branded Pinely hoodie :)

Dear Santa Claus. Please make 2024 a happy year, not just that. Please, we all need this.

Hello, 2024!

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 that 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 2020 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 requirements don'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!