By KAN, 16 hours ago, translation, In English

Hi all!

This weekend, at Nov/29/2020 10:05 (Moscow time) we will hold Codeforces Round 687. It is based on problems of Technocup 2021 Elimination Round 2 that will be held at the same time.

Technocup is a major olympiad for Russian-speaking high-school students, so if you fall into this category, please register at Technocup 2021 website and take part in the Elimination Round.

Div. 1 and Div.2 editions are open and rated for everyone. Register and enjoy the contests!

The problems authors are; amethyst0, eidan, Diegogrc, bensonlzl, Maripium, antontrygubO_o, and KAN. Thanks to antontrygubO_o and 300iq for their help in coordination. Also huge thanks to testers for their invaluable help: Golovanov399, __rkm__, golions, dantrag, postscript, barakraganosungam, Andres_Alumets, firiexp, growup974, Nero, GGMU, K0u1e, Bench0310, dorijanlendvaj, Um_nik, thenymphsofdelphi, Merkurev, the list to be completed later!

Have fun!

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By vovuh, history, 5 days ago, translation, In English

Hello! Codeforces Round #686 (Div. 3) will start at Nov/24/2020 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 ZeroAmbition Stepanova, Mikhail pikmike Piklyaev, Maksim Ne0n25 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!

UPD: I would also like to thank Ivan Gassa Kazmenko for invaluable help with the round preparation!

UPD2: Editorial is published!

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By Ashishgup, 9 days ago, In English

Hi everyone!

I would like to invite you to another one of our rounds, that I set with my friends Jeel_Vaishnav, FastestFinger, Utkarsh.25Dec, the_hyp0cr1t3 and ridbit10

We had two approved contests, but decided to merge it into one with more logical thinking ^_^

The round Codeforces Round #685 (Div. 2) that will take place on 21.11.2020 17:35 (Московское время). If your rating is less than 2100, this round will be rated for you; otherwise, you can participate out of competition.

I would really like to thank my co-setters and:

You will be given 6 problems with one additional subtask and 2 hours 15 minutes to solve them.

Good luck — let the games begin :D

UPD 1: The revised scoring distribution will be: $$$500 - 750 - 1250 - 1750 - (1500 + 1000) - 3000$$$

UPD 2: Editorial — Hope you guys enjoyed the round, we will hopefully be back sometime next year :)

UPD 3: Congratulations to the winners! :D

All Participants:

  1. noimi
  2. I_Love_Convex_Hull
  3. zhouzhendong
  4. dlalswp25
  5. peti1234

Official Participants:

  1. I_Love_Convex_Hull
  2. 221
  3. afterall
  4. Khas_Profit_LLC
  5. imzzy

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By dzyangaraeva, history, 9 days ago, In Russian

Приглашаем учащихся 7-11 классов из России и иностранных государств проявить себя в Олимпиаде по информатике «Innopolis Open».

Этапы проведения олимпиады:

Отборочный этап

Состоит из двух независимых отборочных онлайн-туров, продолжительность каждого 5 часов. В рамках него участникам предлагается по 4—5 задач по информатике.

  • 1-й отборочный тур: 22 ноября 2020 г. 10:00 — 15:00 МСК
  • 2-й отборочный тур: 13 декабря 2020 г. 15:00 — 20:00 МСК

Участник может пройти оба тура, тогда в отборе будет учитываться лучший результат по итогам двух туров. Победители и призеры Innopolis Open по информатике 2020 приглашаются на заключительный этап без отбора.

Отборочный этап также является отбором на Зимнюю школу олимпиадной подготовки по информатике 12—19 февраля 2021.


20—21 февраля 2021 г.

Формат проведения (очно или заочно) заключительного этапа остается под вопросом. В случае очного формата финал будет распределен на нескольких площадках по России и другим странам. Главная площадка будет в Иннополисе.

Innopolis Open 2021 по информатике имеет 2 уровень перечня РСОШ, что позволит победителям и призёрам олимпиады получить преимущество при поступлении в Университет Иннополис и другие вузы.


По всем вопросам:

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By MikeMirzayanov, 10 days ago, In English

Hello Codeforces!

There is something new for you. It seems that now it will be easier to reuse problems from the archive for educational and other purposes.

As you know, problems from past Codeforces rounds can be added to mashups simply by their codes of the form like 1234D (contest ID + problem letter). I myself regularly used this when I taught at Saratov University — it was very convenient to prepare trainings: problems have expected complexity and topics, there are Russian and English statements, you can see the analysis and many solutions. Because of hacks and uphacks, problems in the archive usually have strong tests. Almost all of them are well-prepared. In addition, with the help of user lists, you can easily pick up problems for a group of students that they did not solve.

But there was also an issue: the problems in such trainings were easily googled. And if the majority of adult participants could cope with the temptation to spy on the solution or tests, then beginners sometimes acted dishonestly. I received several requests to come up with something with this flaw. One of the most recent such requests was from Daniel Darooha Sleator ( Thanks, Danny!

So the work seems to be complete. Meet a new feature on Codeforces: now you can rewrite statements for problems in mashups, completely replacing it with your own. More details about the capabilities of the new functionality:

  • a new icon-link appears in the problem list in mashups, by clicking on which there will be a form for creating/editing a new statement;
  • you can use the original text of the statement as a template when writing your own: be very careful — you must exactly repeat all the details of a statement so that problems do not formally differ;
  • this form compares the set of formulas in the original text and the new statement, and if they differ as sets, there will be a corresponding detailed warning;
  • you can specify your own tests from a statement (examples) — here you also need to be very careful so that they are formally correct;
  • if you added examples, then solutions will be judged on them first, and only after on official tests;
  • you can upload your own image files and use them in statements;
  • there is a setting to temporarily disable the use of the new statement (for example, if it is still under development).

Considering that there are already more than 6500 problems in the archive, an excellent perspective opens up for organizing trainings and educational sessions.

I hope this innovation will be useful. Write in the comments what can be improved and/or corrected.


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By pikmike, history, 10 days ago, translation, In English

Hello Codeforces!

On Nov/19/2020 17:35 (Moscow time) Educational Codeforces Round 98 (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 Ne0n25 Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Congratulations to the winners:

Rank Competitor Problems Solved Penalty
1 dlalswp25 6 205
2 Ormlis 6 207
3 Tlatoani 6 214
4 yokozuna57 6 232
5 peti1234 6 235

Congratulations to the best hackers:

Rank Competitor Hack Count
1 racsosabe 80:-17
2 Elo 33:-4
3 peti1234 24
5 xiaofan7 25:-10
605 successful hacks and 888 unsuccessful hacks were made in total!

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

Problem Competitor Penalty
A Geothermal 0:01
B SSRS_ 0:04
C Valera_Grinenko 0:02
D SSRS_ 0:10
E Akulyat 0:38
F 2016wudi 0:58
G rainboy 0:42

UPD: Editorial is out

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By AliShahali1382, history, 13 days ago, In English
$$$~-\text{In the name of God}~-$$$

Hello Codeforces,

I'm glad to invite you to our first contest Codeforces Round #684 (Div. 1) and Codeforces Round #684 (Div. 2) which will be held at Nov/17/2020 17:35 (Moscow time) . The problems are invented and prepared by AliShahali1382, Mehrdad_Sohrabi, and Mohammad.H915. The round is rated for both divisions. You will be given 5 problems and 2 hours 15 minutes to solve them. I recommend you to read all the problems :)

Firstly I'd like to thank isaf27 for coordinating and reviewing the round, as well as helping with many different things.

Thanks for our testers 300iq, growup974, Atreus, Shayan.P, postscript, morzer, BledDest, UnstoppableChillMachine, MAMBA, Prabowo, WNG for testing and giving very helpful advice.

Finally, thanks to MikeMirzayanov for very nice and convenient Codeforces and Polygon platforms.

I hope you all will find the problems interesting. I wish you high scores and good luck!

Scoring distribution:

Div. 1: (500-500)-1250-1750-2500-2500

Div. 2: 500-1000-(750-750)-2000-2500

UPD : Editorial is out.

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By snarknews, history, 13 days ago, translation, In English

Three Regionals Cup — 2020 on the tasks of Moscow Regionals 2020, Belarus and Baltic Regionals 2020, and Northwestern Russia Regionals 2020 is traditionally opened in this year at the Yandex.Contest platform in the format of three virtual contests. On the cup page, you may register for those contests.

You may use the plain Yandex logins or OpenCup or other pre-generated logins (choose the appropriate link at the top of the page).

The regionals results will be included in the cup standings. The result of the Cup is the sum of the Grand Prix 100 (similar to Opencup) scores for all three regionals at 0:00 Nov 30.

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By Miyukine, 5 weeks ago, In English

Hello, Codeforces!

On behalf of Meet IT, I'm glad to invite you to (Codeforces Round #683 by Meet IT (Div1, Div2) which will take place this Sunday (15th November).

The round lasts 2.5 hours and is rated for both divisions.

What is Meet IT?

We are a family of enthusiastic and motivated young people passionate about programming and mathematics. We build a community where people learn together, motivate and help each other.

Please expand the spoiler to find out more about us.

Meet IT family members worked hard over the last few months to provide you with our favourite challenges we came up with. We hope that you will enjoy them as much as we did :)

We are enthusiasts of short and clear problem statements, strong pretests, and happy participants!

Every effort is valuable for us, and therefore I'd like to thank everyone who has contributed to the creation of this round:

  • Co-founders of Meet IT: Mateusz and Paweł for guiding Meet IT projects and making things happen.
  • Maciej, Piotr and Michał. Without their commitment in early years of Meet IT the Family would not exist.
  • Round coordinator antontrygubO_o for endless discussions about the problemset.
  • pawelek1 for coming up with problem ideas.
  • tnowak for massive support in the work on harder tasks.
  • staniewzki and jjaworska for developing the most challenging problem of the round.
  • pasawicki for interest in tennis which was an inspiration to one of the problems, and for preparing another problem.
  • kamilos1089 for preparing a problem quickly and diligently.
  • Mohamed.Sobhy for coming up with a nice easy problem and preparing it.
  • Gajowy for enduring my repetitive reminders to prepare a problem.
  • flaviu2001 for his excitement about the round after testing and help to improve the shortcomings.
  • _h_ for help in providing a high-quality editorial to one of the problems.
  • dorijanlendvaj for firmly complaining about one edgy problem that pushed us towards substituting it with a better one.
  • kobor for providing beautiful pictures for the removed problem. :(
  • Devil for coming up with the substitution problem :), without you we would be still finalizing the problemset.
  • arsijo for paying special attention to statements clarity during testing.
  • zdolna_kaczka for winning the virtual contest among testers.
  • Anadi for inventing an alternative solution to one of the hard problems as well as his kind comments and suggestions.
  • Farhan132 for his ideas on how to improve the problemset in the second division.
  • Linkus and RemiliaScarlet for showing the need to reduce the round difficulty a bit.
  • The Army of Testers: ffao, proggerkz, postscript, Whistleroosh, ijontichy, growup974, AwesomeHunter, H4CKOM, TeaTime, hg333.
  • MikeMirzayanov for the amazing platforms Codeforces and Polygon.
  • all the people who strive for Meet IT Family to grow.
  • Last but not least, my dear wife Marta who was very supportive throughout the long process of preparing the round.


I'm also thankful for Swistakk for testing the round thoroughly. Sorry for forgetting! I wrote the announcement sometime before that. I totally don't get why he got downvoted, please give him as much contribution as he deserves!


There will be $$$6$$$ problems in each division, with $$$4$$$ problems shared. One of those will have a subtask!

We strongly recommend to read all the problems. You have been warned :)


Div 2: 500 — 1000 — 1250 — 2000 — 2250 — (2500 + 750)

Div 1: 500 — 1000 — 1250 — (1750+750) — 3000 — 3000


We would like to host a stream shortly after the contest. We haven't yet figured out the details, you can expect some backstage stories about the round, Meet IT, and casual discussion. Stay tuned!


We've added the link to the stream which shows on the Streams pannel on the right. You're more than welcome to join our stream


Editorial is ready!


We hope you've all enjoyed the round despite some minor issues. Congratulations to the winners!

Division 1:

  1. Um_nik (the only person to solve all problems!)

  2. ecnerwala

  3. Benq

  4. ainta

  5. ksun48

Division 2:

  1. shb

  2. jz_597

  3. 1700012703

  4. XueYJ

  5. zybkl

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