Hello Codeforces! We (dqa2020, Xiejiadong, xryjr233, ZZZZZZZZZZZZZZZZZZ and me) are excited to invite you to take part in Codeforces Round #664 (Div. 1) and Codeforces Round #664 (Div. 2), which will happen on Aug/12/2020 17:35 (Moscow time).

Huge thanks to:

• 300iq for coordinating our round.
• triple__a for answering my questions about contest preparation.
• dysyn1314, jhdonghj112, MiFaFaOvO for providing and discussing ideas.
• gamegame, KonaeAkira, gtrhetr, MiFaFaOvO, mayaohua2003, crasysky, aryanc403, RainAir, dqa2020, diamond_dick, naitir, Juanzhang, _Aaryan_, GavinZheng, Taran_1407, yijan, hwa1996, jhdonghj112 for testing and providing feedback.
• MikeMirzayanov for Codeforces and Polygon platforms.

There will be 5 problems in Div.1 round and 6 problems in Div.2 round. You'll be given 2 hours to solve them.

The story of this round is about that man. Instead of displaying his name, I prefer telling one of his legends (or joke):

"I have a 'friend', who makes lots of money every day, earning a billion in the blink of eyes. With a wave of his hand, OIers all over the world will follow him."

You can post your guesses in the comments.

UPD: Score Distribution:

• Div.1: 500 — 1000 — 1500 — 1750 — 2500
• Div.2: 500 — 750 — 1000 — 1250 — 1750 — 2250

Good luck!

Henlo Codeforces! ^_^

I invite you to participate in Codeforces Round #663 (Div.2) taking place on Aug/09/2020 17:35 (Moscow time). The round is rated for users rated less than 2100, while other users can participate non-competitively.

The round features five problems, and you have 2 hours to solve them. There may, or may not, be an interactive problem; regardless, you should know how to deal with them.

I would, now, like to thank–

• antontrygubO_o for coordination and manipulation.
• AwakeAnay, AsleepAdhyyan, and RestingRajarshi for always listening to my problem ideas.
• BRCode for making 3b1b-style video editorials of the problems!
• Aggu_01000101, Devil, Dragnoid99, NOOBxCODER, NooB_MB, Osama_Alkhodairy, Prabowo, Taran_1407, Tarrus, Tathagat_shah, TheOneYouWant, Utkarsh.25dec, Vivek1998299, _Aaryan_, aryanc403, dorijanlendvaj, growup974, hugopm, knightron00, kshitij_sodani, ltc.groverkss, nishkarsh, rohamizadidoost, shash42, socho, talibmohd, and thenymphsofdelphi for testing!
• MikeMirzayanov for Codeforces and Polygon platforms– Polygon is truly remarkable!

Please do not mind the long list of testers (I had to write code to tag everyone here) since the problem set changed significantly after the first round of testing.

We will announce the scoring distribution shortly. The scoring distribution is 500–750–1250–2000–2750.

Good luck, and stay safe!

UPD: Editorial

Here are video editorials by BRCode:

• Problem A and B
• Problem C
• Problem D
• Problem E

UPD2: Finally, congratulations to the winners!

Div. 1:

1. tmwilliamlin168
2. risujiroh
3. fanache99
4. LayCurse
5. Heltion

Div. 2:

1. 00000010100001100111
2. PouyaNavid
3. 420iq
4. I_want_2100
5. silxi

Hello, Codeforces!

74TrAkToR and I are glad to invite you to our Codeforces Round #662 (Div. 2), which will be held at Aug/07/2020 17:35 (Moscow time). The round will be rated for all the participants with rating strictly less than 2100.

The problems were created and prepared by AlFlen and 74TrAkToR. We would also like to thank everyone who helped us a lot with round preparation.

• Our coordinator 300iq for patience and help with ideas.
• Our dear friend lapochka_queen for the idea of one of the tasks.
• Our testers isaf27, Astralpirate, sava-cska, Siberian, voventa, romanasa, antontrygubO_o, DIvanCode, _overrated_, Mlxa, gerasikov.fml31, Prokhor08 for high-quality testing and valuable advices.
• And our team testers: ainta (ksun48, 300iq) и акацуки (p1rattttt, kostyamyasso2002) for proper testing.
• MikeMirzayanov for amazing Codeforces and Polygon platforms.

On the round you will be asked to help main ponies from My Little Pony animated series (Fluttershy, Applejack, Twilight Sparkle, Pinkie Pie, Rarity, Rainbow Dash) and to solve 5 problems, one of which has two subtasks. You will have 2 hours to solve them.

Score distribution will be announced shortly before the round.

UPD: Score distribution: 500 — 1000 — 1500 — 1750 — (1500 + 1500)

UPD2: Editorial

We wish you good luck and high rating!

Hello! Codeforces Round #661 (Div. 3) will start at Aug/05/2020 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: Huge thanks to Ivan Gassa Kazmenko for testing the round and fixing some issues with statements and the round in general!

UPD2: Editorial is published!

Hello everyone!

Finally published the second part of the lesson about the segment tree (first part is here).

Subtitles are not ready yet, we will add them later.

In this lesson, we will learn how to process mass changing operations on a segment tree.

More about EDU section you can read in this post.

Hope it will be helpful, enjoy!

Hello, Codeforces!

We (Denisov, Karavaev1101, perekopskiy) are glad to introduce to you Codeforces Round #660 (Div. 2), which will happen on Jul/30/2020 17:35 (Moscow time). The round will be rated for the participants with rating lower than 2100, although higher rated users are more than welcome to take part out of competition.

Huge thanks to those who helped make this round possible:

• adedalic for amazing coordinating and reviewing the round, as well as helping with many different things.
• Karavaev1101, perekopskiy for creating problems and preparation.
• dorijanlendvaj, Tlatoani, BOGDAN_, gachiBASS, pikmike, antontrygubO_o, Osama_Alkhodairy, growup974, errorgorn, ilyasiv, Tathagat_shah, ermakov22 for testing and providing invaluable feedback.
• MikeMirzayanov for Codeforces and Polygon platforms.

There will be 5 problems and 2 hours to solve them.

We really hope you enjoy our first contest!

UPD: Here is the score distribution:

750—1000—1500—2000—2750

UPD: Editorial is published!

UPD: Congratulations to the winners!

Div. 1:

1. neal

2. Um_nik

3. heno239

4. jiangly

5. dreamoon_love_AA

Div. 2:

1. okikust

2. SpatialMovement

3. laralalala

4. KD-Tree

5. Mai_madarchod_hu

Hello Codeforces!

On Jul/29/2020 17:35 (Moscow time) Educational Codeforces Round 92 (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!

We hope you’ve been doing well these past couples of weeks.

This week, we wanted to share a blog post about two of our students. As you might know, Harbour.Space has a unique approach to education — besides classwork and exams, we encourage our students to develop their skills with hands-on projects or even create their startups, so that they’re ready for the workforce when they graduate.

That’s exactly what Jonathan and Khaled, two of our Data Science students, did.

After working hard on their Machine Learning-based startup, they were selected by the European Organization for Nuclear Research (CERN) for a 5 Week Student Entrepreneurship Programme, and are now preparing to travel to Geneva in October. We summarized the story of how they went from data scientists to startup founders in this article.

We hope it inspires you to pursue your passions, and work collaboratively to improve the world for those around you.

Good luck on your round, and see you next time!

Congratulations to the best hackers:

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

UPD: Editorial is out

Hello!

This is just a small announcement of the latest updates in EDU. New features:

• Now you can view other's solutions if you have solved the problem yourself. I think this can be very useful. When I I was a contest participant, I always looked as others who are more experienced than me wrote this or that code. Often found something interesting and useful.

• Now you can view tests for problems in the format, as it is done in the archive in the main section of the website.

• Now you can hack other people's solutions if you have solved the problem yourself. And fun and will improve tests of these problems.

More about EDU section you can read in this post.

And what are your wishes for functionality? I remember about filtering the results of practice by friends.

UPD: And for all practice contests I have set a policy of "use time limits scaling policy". This is a recent innovation — you can use it for educational contests, the system will automatically increase the time limit for some languages. Recommended for use only in educational contests.