Hello, Codeforces!

I'm glad to invite you to Codeforces Round 637 (Div. 1) - Thanks, Ivan Belonogov! and Codeforces Round 637 (Div. 2) - Thanks, Ivan Belonogov!, which will be held on Apr/23/2020 17:45 (Moscow time). The round will be rated for both divisions (I hope).

All problems were written and prepared by Aleksey Aleks5d Upirvitskiy, Alexey alexX512 Perevyshin and by me, Denis I_love_myself Sapozhnikov. Thanks to 300iq, voidmax, ashmelev, Akulyat, okwedook, Minnakhmetov, divanik, Zakoden, Jostic11, Nakinamo, 4qqqq и allisyonok for testing problems and good advice, isaf27 for round coordination and help with preparation and MikeMirzayanov for great systems Codeforces and Polygon.

You will be given 6 problems in both divisions and 2.5 hours to solve them. Please, read all the problems. Good luck, have fun and I wish everyone high ratings!

The scoring distribution will be announced closer to the beginning of the round.

Did you notice unusual in the title of the round? Here is the message from MikeMirzayanov:

With this round we want to convey fiery greetings and once again say thanks for the support to Ivan Belonogov Belonogov. And it’s not only about his significant gift on the 10th anniversary of the platform. Starting to participate in 2011, Ivan has come a long way from a participant in Div. 2 rounds to the international grandmaster, became the ICPC world champion as part of ITMO team. Such a striking motivational example of a success story! Thank you, Ivan!

UPD1: The scoring distribution will be:
Div1: 500-750-1250-1750-2250-3000
Div2: 500-1000-1500-1750-2250-2750

UPD2: Due to technical problems, the round is delayed by 10 minutes. Sorry!

UPD3: Editorial is published!

UPD4: This was our first round and we made mistakes wherever possible. Sorry for this. We decided to make Div1 round unrated, Div2 round is still rated. You can read more details in this post. We hope that you enjoyed solving interesting problems and did not pay attention to our mistakes.

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Hello! Codeforces Round 636 (Div. 3) will start at Apr/21/2020 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third 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 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of my work. Thanks to my good friends Daria nooinenoojno Stepanova, Mikhail awoo Piklyaev, Maksim Neon Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round. Also thanks to Artem Rox Plotkin and Dmitrii _overrated_ Umnov for the discussion of ideas and testing the round!

Good luck!

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UPD: Also thanks to Sakhiya07, infinitepro and ma_da_fa_ka for testing the round!

UPD2: Editorial is published!

Riichi...Tsumo! 2 han 2000 points!

Hi! Have you ever heard of the game called Mahjong Soul? It is a Japanese Mahjong game that is famous for the adorable characters.

We are excited to invite you to take part in Codeforces Round #635, where you can help the characters in trouble. This round will be held on Apr/15/2020 17:35 (Moscow time). Most importantly, it is rated for both divisions!

Each division will be given 6 problems and you will have 2.5 hours to solve them. An interactive problem may be found in this round. If you are not familiar with interactive problems, you can learn about them here.

The problems were prepared by EternalAlexander, ustze and me Sulfox.

We sincerely thank isaf27 for reviewing and coordinating the round, and MikeMirzayanov for providing such a great contest preparing environment.

Also thanks to the following testers:

• ouuan, QAQAutoMaton, PinkRabbitAFO, Dilute for testing at first and polishing the statements.

• 300iq, HIR180, gisp_zjz, Cekavis, Sad_reacts_only, mmello, firi., StudyingFather, Clever_Jimmy, Polaris_Dane, spookywooky for testing and providing some suggestions.

Good luck to all the participants! Oh, one more thing, you can enjoy Mahjong Soul here!

UPD 1: The scoring distribution will be:

• Div.1: 500 — 750 — 1500 — 2250 — (1750 + 1500) — 3250
• Div.2: 500 — 1000 — 1500 — 1750 — 2500 — 3250

UPD 2: Protips:

• There will be a sticker in each problem statement except 1F. If you are not interested in the story of the characters, you can skip the sentences above the stickers.

UPD 3: Congratulations to the winners!

• Div.1

1. boboniu (First to solve 1E!)
2. maroonrk (First to solve 1F!)
3. Golovanov399 (First to solve 1B!)
4. Endagorion (First to solve 1D!)
5. FizzyDavid
6. faebdc
7. lzr_010506
8. yosupo
9. Isonan
10. Um_nik

• Div.2

1. Bojangles (First to solve 2E!)
2. JbopkynyRubkLuxSR
3. 01191020csl
4. DreamLoIita
5. hfyzw
6. -45
7. soltanmv
8. Aquaa
9. 18101130I3 (First to solve 2C!)
10. DeD_TihoN

UPD 4: Thank you all for joining us! Editorial is out!

Dear Codeforces,

Happy 10th anniversary from Yandex! Over the past ten years, you have become more than just a platform for competitions having developed into a community hub where people can socialize, discuss interesting tasks and make useful contacts — a large part of life for every competitive programmer. Keep it up – continue growing and developing!

Codeforces, with its immense contribution to competitive programming, is very important to the entire community of sport programmers around the world. Yandex actively supports programming contests and competitions and appreciates what Codeforces does, having benefitted from a large number of talented employees with a background in competitive programming. It wouldn’t be the same without this community.

We would also like to use this opportunity to thank MikeMirzayanov for Polygon, which has become a traditional feature in most of our competitions.

P.S. It is an official account of Yandex. Follow us on Codeforces and stay tuned!

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Hello! Codeforces Round 634 (Div. 3) will start at Apr/13/2020 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third 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 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of my work. Thanks to my good friends Daria nooinenoojno Stepanova, Mikhail awoo Piklyaev, Maksim Neon Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round. Also thanks to Artem Rox Plotkin and Dmitrii _overrated_ Umnov for the discussion of ideas and testing the round!

Good luck!

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UPD: Also thanks to ma_da_fa_ka and infinitepro for testing the round!

UPD2: Editorial is published!

내가 돌아왔다! (Hello, Codeforces!)

I am thrilled to introduce you to Codeforces Round #633. Followings are basic information:

• This contest will take place on Apr/12/2020 17:05 (Moscow time).
• The round is rated for all participants who can understand this announcement. There will be two divisions.
• There are 5 problems in each division and you will have 2 hours to solve it.
• Score distribution will be announced later.

Followings are contributors:

• Authors: McDic (All Div.2 problems and Div.1 D), tzuyu_chou (Div.1 E)
• Coordinator: antontrygubO_o
• Testers:
  • Phase 1 (When this round was supposed to be Div.2): dorijanlendvaj, SoulTch, pajenegod, AryaPawn, Rahul, mcdx9524, 300iq, Nemo, tfg
  • Phase 2 (Before we knew that old Div1D can be easily googled): Um_nik, Ari, benson1029, khulegu, Sad_reacts_only, TselmegKh
  • Phase 3 (Before few days to the round): mohammedehab2002, Kuroni, xuanquang1999, JettyOller, enoone
• CF Admins: Thanks to MikeMirzayanov and his team for maintaining Codeforces and Polygon. I would like to say same sentence again as I said in my last round's announcement. Polygon is super great platform to prepare sports programming contest.

Followings are some fun things:

• antontrygubO_o is the most intense coordinator I have ever met. He rejected lots of my problems. Some example of rejection comments are below:
  • This problem is too standard.
  • Don't ask people about theorem and formula.
  • This problem is appeared in recent ptz camp with higher constraints.
  • I don't like it.
  • Rock Scissor Paper makes statement messy, because some people don't know about it.
  • D2A should be easier than this one.
  • I've found generalized version of this problem in POI.
  • Isn't this obvious?
  • Your proof is wrong.
  • This problem is not very interesting.
  • This is too (censored).
• After lots of rejections, I tried my best to make problems to be interesting. I hope you like at least some of my problems.
• This round was originally supposed to be rated for Div.2 only. However, after we completed Phase 1 testing, we found that my round is too hard for Div.2, so we added more problems and made this round to be Div.1.
• In this round, statements will be even shorter than last contest.
• Even for some of problems which my coordinator approved, there were critical issues that made problems to be excluded. I will introduce some of my rejected problems which won't be used anymore in another post, after this contest ends.

I hope everyone can enjoy my third contest. Thanks in advance!

Followings are updates:

1. Score Distribution:
   • Div.1: 500 1000 1500 2000 2750
   • Div.2: 500 750 1250 1750 2250
2. I am sorry for such Div2C/Div1A statement issue. We fixed that immediately after few minutes to the round, but we should have announced it.
3. Editorial is posted: https://codeforces.net/blog/entry/75913
4. I opened new mashup for excluded problems. These problems are originally approved by antontrygubO_o but excluded for some reasons.
   1. Binomial Determinant is old Div1D. We found this problem on google so we removed this.
   2. Divisible Xor is old Div1A. But since we made 3 xor problems, we removed this one.

Followings are winners:

<Div.1>

1. tourist
2. Radewoosh
3. TLEwpdus
4. ecnerwala
5. gisp_zjz
6. Maksim1744
7. scott_wu
8. Petr
9. jiangly
10. maroonrk

<Div.2>

1. Nachia
2. MyAngelBakapiano
3. 0.142857
4. kekekeke228
5. LeierKing
6. Ka_ng_Hyeon
7. Anithas_love_for_tries
8. confeito
9. TOPCYBERFLOWER
10. wattaihei

Hello Codeforces!

On Apr/10/2020 17:35 (Moscow time) Educational Codeforces Round 85 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Neon Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Our friends at Harbour.Space also have a message for you:

Hey Codeforces,

What a crazy month it’s been!

We hope you and your loved ones are taking care of one another and catching up on everything you didn’t have time for on your normal schedules.

We’ve temporarily closed our physical campus and moved our classes online. It’s not as bad as it sounds though — we’ve always known the future is digital, so we were already preparing for this moment. Either way, we’re back.

SPECIAL PRIZE FOR EDU ROUND 85

This digital transformation opens the door to some pretty awesome new opportunities that allow us to get closer to our community, no matter where in the world they are.

That’s why we have a special prize for the next Educational Round, a space in a course of your choice from our Computer Science or Data Science Programs. You’ll be able to study online with us for 1, 2 or 3 weeks under some of the best Data and Computer Scientists in the world, with all fees on us.

The prize will be for the top 3 users who have confirmed their desire to participate in this competition. To sign up for it, leave us your handle in the form below.

Enrollment in the course you will participate in will depend upon the availability of the course, as well as the prerequisites required for that course.

We hope this provides you with extra incentive for the round, and we’re looking forward to seeing some of you soon!

FULL SCHOLARSHIPS FOR BACHELOR’S PROGRAM

On a different note, we’re happy to announce that we’re offering full scholarships for the most talented high school students who want to study in our Computer Science or Data Science Bachelor’s programs at our university.

At the moment, we have two types of scholarships available:

The Full Scholarship. For the best of the best — all tuition costs are covered by the university. Just show up and show us what you’ve got.

The Co-Creator Scholarship. For those who want to help us change education — all tuition costs are covered by the university, and in addition to your studies, you get to help the Harbour.Space team on their mission for 4 hours/day, getting valuable practical experience in the field and working in a team with some of the brightest young professionals in town.

Fill out the form below and we will contact you for the next steps!

Looking forward to seeing you kick some ass in the next Educational Round, and stay safe!

Best, Harbour.Space University

Congratulations to the winners:

Congratulations to the best hackers:

And finally people who were the first to solve each problem:

UPD: Editorial is out

Hello, Codeforces!

I'm glad to invite you to my first round Codeforces Round 632 (Div. 2), which will take place on Apr/08/2020 17:35 (Moscow time).

"Thank you" time!

I'd like to thank:

• antontrygubO_o for coordination of the round.
• 300iq, nvmdava, aryanc403, antoshkin, lkosten, pavel-afonkin for testing and useful advices.
• MikeMirzayanov for great systems Codeforces and Polygon.
• CuriOusQuOkka for listening to my mumble while I came up with tasks.
• antontrygubO_o for coordination of the round (again).

You'll be given 6 problems and 2 hours to solve them. Point distribution will be announced as soon as possible.

Hope you will enjoy the round! Good luck and have fun!

P.S: YES, IT'S RATED.

UPD: Score distribution: 500 — 750 — 1250 — 1750 — 2000 — 2500.

UPD2: Congratulations to the winners!!

Div2:

• AnsuFati
• gmusya
• Alex_Wei
• sweetwrapper
• wjli

Wow, there is no unrated in div2 round!

All:

• jiangly
• jqdai0815
• Roundgod
• summitwei
• tmwilliamlin168

UPD3: I will post solution in several hours.

UPD4: It was very long several hours, I'm very sorry for that :( Editorial