Hi Codeforces!

I’m thrilled to invite you to Codeforces Round 638 (Div. 2), which will take place on 01.05.2020 17:35 (Московское время). It's rated!

There will be 6 problems, and you will have 2.5 hours to solve them. All the problems are written and prepared by me.

Huge thank you to everyone who made this round possible:

• adedalic for awesome coordination, guidance, advice, and more <3

• dragonslayerintraining for extensive testing, feedback, and buffing tests

• awoo, walnutwaldo20, Benq, hocky, Geothermal, isaf27, McDic, bfs.07, iankury, songsinger, Agnimandur, dcshin, chrome, Rods, brian, and Khairy for taking the time to test and provide invaluable advice

• MikeMirzayanov for wonderful platforms Codeforces and Polygon (preparing problems on Polygon is fun!)

• You, for participating!

I hope everyone will enjoy the problems! I have tried to make short statements and strong pretests. Hopefully this contest will be even better than my previous one :)

UPD: Thanks also to darnley for proofreading statements and suggestions!

UPD: The scoring distribution is standard: 500 — 1000 — 1500 — 2000 — 2500 — 3000

UPD: Editorial is here

UPD: Congratulations to the winners! I hope most of you enjoyed the round. If you have any questions, feel free to ask in the comments.

Overall Winners:

1. dreamoon_love_AA

2. KrK

3. peti1234

4. antontrygubO_o

5. I_love_Tanya_Romanova

Div 2. Winners:

1. Fecto_Elfilis

2. czyarl

3. NishimiyaShouko

4. PureVessel

5. Neraxis

Hi Codeforces!

The Hash Code 2020 World Finals took place on Saturday, April 25! 45 teams representing 22 countries qualified to participate in this virtual event. During four hours of intense competition, finalist teams made 2141 submissions. It was a close competition!

Congratulations to the winners of Hash Code 2020!

• 🥇 Past Glory, score: 6,696,284
• 🥈 simplicissimus, score: 6,478,445
• 🥉 ✷code, score: 6,238,462

View the public scoreboard here.

For those who didn't participate in the World Finals but want to try this year's challenge, we have re-opened our competition platform for an Extended Round, which can be accessed via the Judge System. Please note that you must have registered for the Hash Code 2020 Online Qualification Round and formed a team in order to take part in the Extended Round. The Extended Round will stay open until May 14 at 17:00 UTC.

As an additional bonus, we have added the problem from Hash Code 2019 Online Qualification Round on Kaggle! The problem can be accessed via the Playground Code Competition. Please note that you need to sign up for Kaggle to compete. The round will stay active until July 27 at 23:59 UTC. After this deadline it will be still possible to make submissions, however, the leaderboard will not be updated.

Have fun coding!

Your Hash Code Team

Hi!

As many have noticed, sometimes problem ratings were assigned in a strange way that was not consistent with expectations. For example, ratings for complex problems of Div3 rounds were often overestimated. This was mainly due to the fact that high-ranking unofficial participants did not try such problems. It turned out that despite the high rating of a participant, a problem is not solved by the participant, and this fact raised the rating of the problem. It is not entirely correct to take into account only official participants since ratings for difficult problems are sometimes more accurately determined by unofficial participants.

Somewhere in the comments, I've read that problem ratings are set manually. Of course, this is not so. The process is automated, but I start it manually (I will fix it somehow).

I changed the formulas for calculating problem ratings, now they slightly better correspond to expectations. New problem ratings are already available on the website. I don't think they are perfect (but I hope that they are much better). If somewhere ratings obviously are wrong — it would be great to see such examples in the comments.

Thanks!

UPD 1: Thank you for examples of unexpected problem ratings. I'll try to fix them (I don't think that it is possible to fix all of them without manual work) and return with an update.

UPD 2 [May, 2]: I made another attempt to adjust the coefficients, to take into account some facts differently. The ratings are recalculated again. I carefully went through most of the comments and indicated new ratings. Now it looks a little better. I afraid, there are still some issues with some problems. Try to find them and demonstrate them in the comments. Thanks!

Hello Codeforces!

On Apr/26/2020 17:35 (Moscow time) Educational Codeforces Round 86 (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.

Special thanks to Mikhail darnley Dvorkin for helping in round preparation!

Good luck to all the participants!

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

Hi Codeforces!

You really went for it in the last Educational Round! We had an all-time high participation of 21750 people :) We are happy to support such an awesome community, and look forward to growing these numbers in the future!

We are searching for diamonds in the rough — driven, talented humans, passionate about technology and design, undefined by nationality, gender and cultural background. We know that no diamond is born polished, so our mission is to identify and support as many talented young individuals as we can, so that they can fulfill their potential and secure the future they deserve.

If you are graduating or have already completed a bachelor's degree, we are waiting for your applications for fully-funded Master's degree scholarships by the link below.

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 writing this post to make comments about a situation on the last Codeforces Round and tell the bad news. We did many mistakes and due to them, the round caused much dissatisfaction from the Codeforces community.

Some of our mistakes:

1. Some stupid mistakes in the statements.

2. In the problem D2E/D1C the numbers $$$d_i$$$ were not necessarily sorted, but there were no pretests for that.

3. The checker of the problem D2F/D1D didn't check one of the requirements and due to that $$$8$$$ solutions failed on pretests during the system testing.

4. After the editorial was posted the mistake was found in the main solution of the problem D1E (more details here). Now we don't know the correct solution.

So, the round was very unsuccessful, I'm very sorry about this. I will make conclusions and will try to avoid such situations later. Please, treat with respect to the authors, even in such a bad situation, a big work was done and I hope you enjoyed the problems.

Now about the bad news: due to the wrong checker and the wrong solution, we decided to make Div1 round unrated, Div2 round is still rated, Sorry for all, who had a big positive rating change.

Also, we are making a D1E problem-solving challenge: if you have an algorithm, that can be proven, please share your approach.

So, that was all news, sorry again,

Ivan.

P.S.

If you are angry now, you can use this post to set a dislike, please don't dislike an announcement/editorial.

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!