Hello again, Codeforces!

We are glad to invite you to Mathforces Thinkforces Good Bye 2019, which will take place on Dec/29/2019 17:05 (Moscow time).

Some information about the round:

• Rated for all participants!
• 3 hours!
• No subtasks!
• There will be an interactive problem in this round. You can read the guide for interactive problems here
• Editorial will be published right after the system testing

All problems in this round were prepared by us, antontrygubO_o and kefaa2. We worked on this round for a long time and tried to make all the problems very interesting. We hope that you will enjoy the round!

We would like to thank:

• arsijo for coordination of the round
• GandalfTheGrey for the idea of solution of one problem
• gepardo for useful discussions of the problems
• farmersrice, Danylo99, 300iq, burstintotears, danya.smelskiy, opukittpceno_hhr, Jeel_Vaishnav, StasyaCat, shoemakerjo, spar5h, RomaWhite, Juve45, Stepavly, Supermagzzz, WhaleVomit, AdvancerMan, Andres_Alumets, -lwl- for testing and invaluable advices. If this a record number of testers? O_o
• MikeMirzayanov for great systems Codeforces and Polygon
• Entire Codeforces community for making this year so fun!

The number of the problems and point distribution will be announced shortly before the round (or earlier).

Good luck and have fun!

UPD1: Using opportunity, we would like to advertise the match between tourist and Um_nik, which will start in half an hour after this round ends.

UPD2: The last contest of the decade on Codeforces will feature 9 problems .

Score distribution:

500 — 1000 — 1500 — 2000 — 2750 — 3250 — 3750 — 4000 — 4500

UPD3: Editorial

UPD4:

Congratulations to winners:

1. Radewoosh
2. Um_nik
3. yosupo
4. FizzyDavid
5. ksun48
6. isaf27
7. Petr
8. WZYYN
9. AndreySergunin
10. saba2000

Hey all!

We've created a new 1v1 programming contest format: Lockout. Like in most contests, each round has a set of problems and contestants work to solve them as quickly as they can. In Lockout, however, contestants compete head-to-head and only the first contestant to solve each problem gets the points. Contestants can work on problems in any order, so speed and strategy are crucial to avoid getting sniped! The head-to-head action also makes the contest much more exciting for viewers.

We ran the first edition of Lockout at TCO Finals last month as a double-elimination bracket tournament. All of the finalists who were available competed (and even some of the problem writers) and we got to see a lot of exciting back-and-forth matches! As you can see though, there's still one set left to play. So we'll be streaming Grand Finals of Lockout 0 featuring tourist vs. Um_nik at 9:30 AM PST, right after the end of Good Bye 2019 on Sunday. Tune in at twitch.tv/ttocs45 to see the finals with live commentary from myself and ecnerwala! We'll watch the screens and scoreboard live, talk to tourist and Um_nik to hear their strategies, and even have a special exhibition match afterwards. :)

The finals will feature five problems with the following scoring: 100 — 200 — 300 — 400 — 500. There are 1500 points total, so the first person to get 800 or more points will be the winner. What strategies would you try? Start with A and B, then jump to E if you get them both? Or start with E, and try to win outright with either C or D afterwards? Read all the problems first and then choose a strategy based on what your opponent solves first? Let us know your thoughts!

Pay attention to the unusual round start time.

UPD: We cannot determine difficulty of some problems thus we recommend you to read all problems and think about each of them.

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Hello! Codeforces Round #611 (Div. 3) will start at Dec/28/2019 20:05 (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.

Good luck!

I also would like to say that participants who will submit wrong solutions on purpose and hack them afterwards (example) will not be shown in the hacking leaders table.

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UPD2: Editorial is available!

Hello, Codeforces!

Before the New Year, I, MikeMirzayanov and geranazavr555 want to present you a new update of talks interface. Now you can set bound to the group of people who can send you messages. Namely, you can prevent user from messaging you, hide user’s messages or both.

Also, some users had trouble getting too many unnecessary messages. Now you can choose the minimal rating of people who can send you messages. This rule will not be applied to existing dialogues. And your restrictions will not affect your friends and administrators of Codeforces.

Hello Codeforces!

On Dec/27/2019 17:40 (Moscow time) Educational Codeforces Round 79 (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 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!

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

Hi Codeforces!

Last spots available for Mike Mirzayanov's course Advanced Algorithms and Data Structures, which will take place in Barcelona, from the 6th to 24th of January, 2020.

The course will consist of three weeks of training, 5 training days each week. The program includes daily lectures and practical exercises. It will be quite educational, insightful and entertaining!

Remember, there’s a special price of 1,000 EUR* for all Codeforces users.

* The cost does not include travel or accommodation.

If you’re interested, send us a message at hello@harbour.space and we will guide you through the next steps.

UPD: Editorial is out

Do you already have a New Year's mood?

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 2016 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. 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!

Hello, Codeforces!

HSE Faculty of Computer Science and Yandex launch registration for the 3rd International Data Analysis Olympiad (IDAO 2020). The platinum partner of IDAO 2020 is Qiwi.The Olympiad includes two stages:

Online Stage (Jan 15 – Feb 11, 2020):

• Track 1: Traditional machine learning competition on Yandex.Contest platform. You will need to make new predictions and upload them to the automatic verification system.

• Track 2: Come up with a solution for the same problem, keeping within a rigid framework of time and memory used.

Final (April 2-5, 2020):

• top 30 teams according to the Online Stage results will be invited to the on-site final, which will be held on April 2-5 in Yandex office, Moscow.

• In the final 36 hours of the competition, participants will try not just to train the model, but to create a full-fledged prototype, which will be tested both in terms of accuracy and performance.

As part of IDAO Final, performances and master classes of world experts in machine learning and data analysis are also planned. Winners and prize-winners of IDAO 2020 will receive valuable prizes and gifts, as well as an advantage in entering Yandex School of Data Analysis and master's programmes at the HSE Faculty of Computer Science.

In 2019, 2187 participants from 78 countries took part in IDAO, and 79 participants from 7 countries met in the final in Moscow.

Learn more: https://idao.world/

Hello Codeforces!

We're glad to invite you to Codeforces Round #610 (Div. 2), which will be held on Dec/24/2019 17:35 (Moscow time). It will be rated for all participants with a rating below 2100. You will be given six problems and two hours to solve them.

The problems were invented and prepared by the team from ITMO University: Supermagzzz, Stepavly, AdvancerMan, unreal.eugene, MikeMirzayanov.

Special thanks to MikeMirzayanov for great systems Codeforces and Polygon and coordinating round preparation.

UPD1: Special thanks to the ITMO University testers team: Nebuchadnezzar, Alexvsalex, Darui99, golikovnik, sharepoLOVEDDD, Mr.Hakimov, zergey.gad, MeowMr, Ilya-bar, 1ncendiary

There is an interactive problem in the round. You can read the guide for interactive problems here.

UPD2: Scoring: 500 — (500 — 1000) — 1750 — 2500 — 2500

We hope you will enjoy the problems. Good luck, wish you a high rating!

UPD3: Congratulations to the winners!

1. Tsypkokokokoko
2. K.Yuuki
3. hyhtsdy
4. ptica
5. junukwon8
6. A_n_o_n_y_m_o_u_s
7. AnotherRound
8. ark_84
9. rtilikay
10. p1rattttt

Editorial will be published soon.

UPD4: Editorial is out!