Hello Codeforces!

We have a pleasure to invite you to Codeforces Round #647 (Div. 1) and Codeforces Round #647 (Div. 2). This round will take place on Jun/04/2020 17:35 (Moscow time). In both divisions, you will have 2.5 hours to solve 6 problems. Three of them will be shared.

The problems for this round were prepared by MicGor, Grzmot, Okrut and me.

We would like to thank everyone who made this round possible:

• Nebuchadnezzar for the excellent coordination;

• Lewin, ALILILILILI-KHAN for invaluable advice and extensive testing;

• KrK, yarek, -is-this-fft-, w0nsh, Marckess, Ari, Fortin, Dup4, kraborak, 300iq, 0ogway, HIR180 for taking the time to test and providing useful feedback;

• MikeMirzayanov for great platforms Codeforces and Polygon.

We hope you will enjoy the problem set! Good luck!

UPD: Score distribution:

• Div 1: $$$500$$$ $$$-$$$ $$$1250$$$ $$$-$$$ $$$2000$$$ $$$-$$$ $$$2500$$$ $$$-$$$ $$$3000$$$ $$$-$$$ $$$3500$$$
• Div 2: $$$500$$$ $$$-$$$ $$$1000$$$ $$$-$$$ $$$1500$$$ $$$-$$$ $$$2000$$$ $$$-$$$ $$$2750$$$ $$$-$$$ $$$3500$$$

UPD: Editorial

UPD: Congratulations to the winners!

Div. 1:

• tourist
• Um_nik
• boboniu
• ecnerwala
• maroonrk

Div. 2

• rainboy
• Charbel11
• firecookie
• _khushi_
• rasengans

Below is a message for you from MikeMirzayanov:

With this round, we want to say thank you to Algo Muse for the significant contribution and support!

Algo Muse is an educational site that conducts online algorithmic contests in pen-and-paper style (no coding). It was originally started to help students prepare for theory qualifiers for graduate admissions. At present, the problems have a broader appeal, though occasionally an odd problem may require knowledge of linear algebra, probability, or group theory. The website is maintained by former students of IIT Bombay. To find out more, visit their website or their twitter account.

Hello!

Together with colleagues Schaffhausen Institute of Technology we hold such an event. If you want to study in Switzerland, then perhaps this is your chance!

— Mike

Hello, Codeforces!

We are thrilled to announce a new SIT STAR Contest by the Schaffhausen Institute of Technology in Switzerland. The winners will have a chance to get a fully-funded Master’s scholarship in Computer Science and Software Engineering.

What is the SIT STAR Contest?

The goal of the SIT STAR Contest is to promote interest in the field of Computer Science and Software Engineering, give students an opportunity to demonstrate their knowledge of programming, and be considered for a fully-funded graduate scholarship. You can apply to the contest here!

The SIT STAR Contest consists of:

1. June 1-7, 2020 | Practice: To familiarize yourself with the testing environment, you will first be granted access to a practice round. You can practice any time from 1st to 7th June 2020. This is an optional step but we highly recommend to take part in it. The results of this round won’t affect the final score.
2. June 17, 2020 | SIT STAR Contest: The final round will take place on 17th June. The participants will be given 4 hours to complete it.
3. June — July, 2020 | Interviews and Winners announcement: Top participants with the highest scores will be invited for the interviews with the professors from the Schaffhausen Institute of Technology.

The SIT STAR Contest will include 8-12 problems of various levels of difficulty in algorithmic programming.

Hi everyone!

I would like to invite you to my fourth Codeforces Round, which I have made with my friends FastestFinger and TheOneYouWant. In terms of problems, it is my favorite among all my rounds.

With that said, I bring to your attention our new Codeforces Round #646 (Div. 2) that will take place on May/31/2020 17:35 (Moscow time). 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:

• FastestFinger and TheOneYouWant for helping with preparing problems and giving new ideas.
• antontrygubO_o for coordinating our round
• 300iq, Jeel_Vaishnav, Ari, aryanc403, mahmoudbadawy, Vivek1998299, Uzumaki_Narutoo, Bakry_, smartnj and Nikhil_Medam for testing the problems.
• MikeMirzayanov for Codeforces and Polygon platforms.

You will be given 6 problems and 2 hours to solve them. Scoring distribution will be announced later.

Good luck! :D

UPD: Scoring Distribution: $$$500-1000-1500-2000-2250-3000$$$

UPD2: Editorial (with memes, and detailed explanation) — Hope you guys enjoyed the contest! :D

Hello, Codeforces!

First and foremost, we would like to say a massive thank you to everyone who entered and submitted their answers to the first, second and third Kotlin Heroes competitions which were held previously. Congratulations to the top 3 winners:

Episode 1

1. Petr
2. ecnerwala
3. eatmore

Episode 2

1. tourist
2. eatmore
3. Benq

Episode 3

1. Egor
2. tourist
3. Benq

Ready to challenge yourself to do better? The fourth "Kotlin Heroes" competition will be hosted on the Codeforces platform on May/29/2020 17:35 (Moscow time). The contest will last 2 hours 30 minutes and will feature a set of problems from simple ones, designed to be solvable by anyone, to hard ones, to make it interesting for seasoned competitive programmers. Top three winners will get prizes of $512, $256, and $128 respectively, top 50 will win a Kotlin Heroes t-shirt and an exclusive Kotlin sticker, competitors solving at least one problem will enter into a draw for one of 50 Kotlin Heroes t-shirts.

Registration is already open and available via the link. It will be available until the end of the round.

The round will again be held in accordance with a set of slightly modified ICPC rules:

• The round is unrated.
• The contest will have 9 problems of various levels of complexity.
• You are only allowed to use Kotlin to solve these problems.
• Participants are ranked according to the number of correctly solved problems. Ties are resolved based on the lowest total penalty time for all problems, which is computed as follows. For each solved problem, a penalty is set to the submission time of that problem (the time since the start of the contest). An extra penalty of 10 minutes is added for each failed submission on solved problems (i.e., if you never solve the problem, you will not be penalized for trying that problem). If two participants solved the same number of problems and scored the same penalty, then those of them who had previously made the last successful submission will be given an advantage in the distribution of prizes and gifts.

If you are still new to Kotlin we have prepared a tutorial on competitive programming in Kotlin and a practice round, where you can try to solve a few simple problems in Kotlin. All the solutions are open, which means that you can look at the solution even if you haven't solved the problem yet. The practice round is available by the link.

We wish you luck and hope you enjoy Kotlin.

UPD: The editorial can be found here. Thank you for participation! I hope you enjoyed the problems.

Hello Codeforces!

On May/28/2020 17:35 (Moscow time) Educational Codeforces Round 88 (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!

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

Hey Codeforces!

As you may remember, we have a free webinar series starring our all-star faculty members who share valuable content and insiders’ knowledge that you don’t get to learn about in traditional classrooms.

Join us tomorrow, Thursday, May 28 at 12h (BCN) / 17h (BKK) to watch Sergey Gordeychik, CIO of the Inception Institute of Artificial Intelligence, share his expertise and insights in his “Digital Lockdown: AI against COVID-19” session. Sergey will discuss how AI is being used both positively and negatively during the COVID-19 global pandemic. Tune in for some practical examples of how companies are using AI to innovate and disrupt during a time of crisis, exploring topics like Medical Imaging for CT analysis, diagnosis and mass surveillance.

By participating in this webinar you will get a certificate of participation, a special digital gift from Sergey, and stand a chance to win a FREE 3-week module at Harbour.Space University depending on the availability and prerequisites of the course.

See you tomorrow, and good luck on your round!

Congratulations to the winners:

Congratulations to the best hackers:

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

UPD: Editorial is out

Сoronavirus work, coronavirus school, coronavirus rest, coronavirus time spending, coronavirus contest.

Hello, Codeforces!

Codeforces Round #645 (Div. 2) will start at May/26/2020 17:35 (Moscow time). This round will be rated for the participants with rating lower than 2100. You will have 2 hours to solve 6 problems.

Problems of this round were prepared by Alexey Alexdat2000 Datskovskiy, Ilian ilian04 Andrianov, Vsevolod Sevlll Lepeshov. We have made an effort to create interesting problems, beautiful statements and strong tests. We wish you like problems and your rating increase.

We would like to express our gratitude to:

• MikeMirzayanov for wonderful platforms Codeforces and Polygon!

• 300iq for excellent coordination and helpful advices!

• Thanks to Siberian, mohammedehab2002, Degalat57, Kuroni, pajenegod, hugopm, Dart-Xeyter, alexxela12345, FlameDragon, Monogon, YanikusGG, Mangooste, mbolgov, Ziware, xiryss for delightful testing!

• Special thanks to testers imachug, dorijanlendvaj, antontrygubO_o,_overrated_ and Allvik06 for help in finding errors in problems!

UPD 1: Scoring distribution: 500 — 750 — 1500 — 1500 — 2000 — 2500

UPD 2: editorial

UPD 3: Congratulations to winners!

Top 5 official participants:

Top 5 all participants:

Participants who sent the first correct solution to the problems:

Hello everyone.

In the coming days I will finish the implementation of small changes in the rating calculation for new accounts. Here are the main innovations.

• The rating of the new account will be equal to $$$0$$$ for display (but it will be equal to $$$1400$$$ when calculating rating changes).
• Suppose, after the first round, the participant gained $$$d_1$$$ rating points (remember that the rating was considered equal to $$$1400$$$ in such calculations), then in the rating display after this round $$$500+d_1$$$ will be displayed. Thus, after the first participation, the rating is likely to increase from $$$0$$$ to a value in the region of $$$500$$$ (plus or minus $$$300$$$ approximately).
• Thus, before the second participation, the displayed rating is $$$500+d_1$$$, and the rating for calculating changes is $$$1400+d_1$$$. Suppose a new change is $$$d_2$$$, then the displayed rating becomes $$$500+d_1+350+d_2$$$.
• Thus, before the third participation, the displayed rating is $$$500+d_1+350+d_2$$$, and the rating for calculating changes is $$$1400+d_1+d_2$$$. Suppose a new change is $$$d_3$$$, then the displayed rating becomes $$$500+d_1+350+d_2+250+d_3$$$.
• And so on the first $$$6$$$ rounds. Promotions of the displayed rating will be equal to $$$500, 350, 250, 150, 100, 50$$$ (in total exactly $$$1400$$$). Therefore, after participating in $$$6$$$ rounds, the rating is “stabilized” and the displayed rating will be equal to the one used to recalculate the changes.

Thus, on average, for new accounts at an early stage, the rating will increase, getting closer to the real value. These innovations will help solve several issues:

• We are reducing the starting rating from $$$1500 $$$ to $$$ 1400$$$, so that new accounts make a smaller contribution to the overall rating.
• Currently, especially for inexperienced participants, there is an effect that their rating at the beginning of participation is constantly falling (from $$$1500$$$ to the value that is a true assessment of their skills). This, of course, leads to discomfort: you try, participate, solve some problems, but the rating drops.
• The previous paragraph gives additional motivation to create new accounts. If your current rating is less than $$$1500$$$, then there is a temptation to start from scratch with a fresh account. After the changes, such a start will require a noticeable long-term effort to climb at least your current rating. It seems that the changes should slightly reduce the motivation to create new accounts.

Apparently, a similar idea is used on AtCoder, but I don't know the details.

What do you think about such improvement? Sounds good?

Hello, Codeforces!

We have planned this round as a Div. 4, but testing showed us, that problems are more suitable for Div. 3. Therefore, instead of the previously announced Div. 4 we will hold Div. 3 round.

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Hello! Codeforces Round #644 (Div. 3) will start at May/24/2020 16:35 (Moscow time). You will be offered 8 problems 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 a 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 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 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.

The problems for this round were invented by MikeMirzayanov and prepared by me and Stepavly

Thanks to MikeMirzayanov for platforms and coordination of our work. Thanks to antontrygubO_o, iankury, bfs.07, kocko, Ilya-bar, arsijo, pashka, Gassa, neal, vovuh for help in round preparation and testing the round.

Good luck!

</almost-copy-pasted-part>

UPD: Editorial is out

Hello Codeforces!

On May/17/2020 12:20 (Moscow time) Educational Codeforces Round 87 (Rated for Div. 2) will start.

Please notice the unusual time.

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.

Also thanks to Neal neal Wu for testing.

Good luck to all the participants!

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

Hi Codeforces!

We would like to invite you to a very special webinar called Digital Lockdown: AI against COVID-19, by Sergey Gordeychik, director of our Cyber Security programme.

Sergey is CIO of Inception Institute of Artificial Intelligence, and former CTO at Kaspersky.

During this webinar, Sergey will share his expertise and insights on how AI is being used both positively and negatively, during the COVID-19 global pandemic. Tune in for some practical examples of how companies are using AI to innovate and disrupt during a time of crisis, exploring topics like Medical Imaging for CT analysis, diagnosis and mass surveillance.

Join us on Thursday, May 28th at 12h (BCN) to gain knowledge and deepen your understanding about how we can use AI to solve both operational and societal problems.

By participating in this 1hour webinar you will get a certificate of participation, a special digital gift from Sergey, and have the chance to win a FREE 3-week module at Harbour.Space University, depending on the availability and prerequisites of the course.

Congratulations to the winners:

Congratulations to the best hackers:

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

UPD: Editorial is out