Hello Codeforces! Some months ago I wrote tutorial about basics of recurrent sequences. This blog, my second tutorial is about mine another favorite topic — Invariants and Monovariants. These are useful concepts which will help you solving problems, especially, constructive ones. Even if you are not familiar with this topics, I'm sure that you solved problems with use of invariants and monovariants without knowing it, continue reading, you will understand my point :) There will be hard problems which are based on old IMO problems, so I will try my best to explain solutions of examples.

So, what are Invariants and Monovariants? An invariant is a quantity that doesn't change. A monovariant is a quantity that changes monotically (that is, non-decreasing or non-increasing). Seems simple, yes?

Let's start with a few easy examples which you will better understand its point. I suggest you to try solving problems by your own before reading solution.

Hello Codeforces!

I'm proud to announce that Codeforces Round #457 (Div. 2) will be held on 19 Jan, 14:35 UTC. The problemset is authored by southball, longhuenchan and me (STommydx). It is our first round in Codeforces and hope you all find the problems interesting.

We would like to thank the following people as the round would not be possible without their kind help: vintage_Vlad_Makeev for coordinating the round, Arpa for testing the problems and MikeMirzayanov for the great Polygon and Codeforces platform.

The contest will consist of 5 problems and you'll be given 2 hours to solve them. The scoring distribution will be announced close to the start of the round as tradition.

Let me close this blog post by answering the most frequently asked question in Codeforces. This round is rated for all Div. 2 participants. As usual, Div. 1 participants can join out of competition.

Good luck and high ratings!

UPD:

Scoring distribution: 500-1000-1500-2250-2500

UPD2:

We are extremely sorry for the situation for problem B. We are figuring out a correct solution for problem B. I hope you all enjoy the rest of the problems.

UPD3:

The round is over. Congratulates to the winners!

Div2:

1. ustatt
2. HanaElhami
3. SorryBahadir
4. ntv
5. Om_nik

Div1+2:

1. eddy1021
2. chemthan
3. nhho
4. fmota
5. ustatt

The editorial will be available tomorrow.

UPD4:

Editorial is ready!

Hello Codeforces!

On January 13, 16:05 MSK Educational Codeforces Round 36 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 (traditionally now) rated for Div. 2. It will be held on extented ACM ICPC rules. After the end of the contest you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 7 problems and 2 hours to solve them.

The problems were prepared by Mikhail awoo Piklyaev, Ivan BledDest Androsov, Sergey sslotin Slotin and me.

Good luck to all participants!

I also have a message from our partners, Harbour.Space University:

As we get ready to dive into the second week of the year, we want to update you all on the upcoming Hello India x Russia Programming Bootcamp! So far 25 teams have registered, with more signing up daily.

As a reminder, the boot camp will take place from March 22nd to March 30th, 2018, at the Amrita School of Engineering campus, India. The Coordinator of the Programming Committee, and head coach will be two time ACM-ICPC world vice-champion Gleb GlebsHP Evstropov.

You can find more information and registration, click here

For those of you who need financial support for the boot camp, please fill up the register form, then we will contact you and prepare an official sponsorship request letter for you to present to your University, University’s IT partners or your potential employer.

Congratulations to the winners:

Congratulations to the best hackers:

2023 successful hacks and 1300 unsuccessful hacks were made in total!

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

UPD Editorial

Hello 2018!

If you're still thinking what to do on the eighth day of year 2018, pay attention! The first round for both divisions of the new year starts on January 8 at 17:35 Moscow time (what about other timezones?).

Four important components of Hello 2018 will be the same as in Good Bye 2017:

• Div1 + Div2 combined
• 8 problems
• 2 hours 30 minutes
• Rated

But there will also be a substantial difference:

• Different problems

The problems of this round have been proposed and prepared by YakutovDmitriy, budalnik and myself.

Thanks to everyone without whom this round wouldn't be possible as well: AlexFetisov, Golovanov399, KAN, MikeMirzayanov, PavelKunyavskiy, qwerty787788, VArtem, winger.

Good luck!

Scoring distribution: 500 — 750 — 1000 — 1250 — 1750 — 2250 — 3000 — 3500.

Problem tutorial can be found here.

Congratulations to the winners!

1. Um_nik
2. desert97
3. yosupo
4. dotorya
5. zeliboba
6. FizzyDavid
7. Endagorion
8. .o.
9. SpyCheese
10. Kostroma

Hey, Codeforces!

Hope that the last few days were quite productive for all of you in terms of rest and pleasure. Time to forget about messing with handle colors and get back to coding!

I'm proud to usher the first contest of the year — Codeforces Round #456, which is going to be held on Friday, 17:35 MSK. The round will be rated for all division 2 participants. Anyway, we will be glad to see whoever decides to join us out-of-competition :)

The authors are — GreenGrape and rek (don't worry, it's all about New Year magic ^^). My heartfelt gratitude to Kolya (KAN) for his invaluable help in round coordination, Grisha (vintage_Vlad_Makeev) and Oleg (xen) for testing and correcting statements, Dima (cdkrot) and Gleb (Glebodin) for preliminary solving, AmirReza (Arpa) for close reading and Mike (MikeMirzayanov) for awesome Codeforces and Polygon platforms.

We look forward to brightening up your cold winter evening :)

GL & HF!

UPD. There will be five problems with the following scoring:
500 — 1000 — 1750 — 2000 — 2500
As usual, it's strictly recommended to look through all problems :)

UPD. Please note that the registration was delayed for a few hours.

UPD. The contest is over.
It turned out to be a little bit more complicated than we expected. Still we hope you learned something new :)

UPD. Editorial!

UPD. Congratulations to the winners!

Div. 2:

1. apink
2. anger_of_singledogs
3. stepfather
4. supy
5. Ratel

Div. 1 (unofficial):

1. dotorya
2. ksun48
3. irkstepanov
4. Benq
5. quailty

Happy New Year, Codeforces!

I hasten to wish the whole community (and including me) correct programs, sudden insights, beautiful ideas and interesting problems!

I hope that you have met the new year at least as fun as I am. Have you had enough sleep after New Year's Eve? This year, the traditional post summarizing the past year, I sat down to write only on January 1, 2018. I hope that I will not have to sum up the whole year.

This post is important to me, since it draws a line to all the work done by the Codeforces team and the entire community in 2017. Many thanks to the team: all of the achievements listed below are the result of joint efforts. We did an excellent job! The community must know its heroes. In 2017, MikeMirzayanov, KAN (problem coordinator), vintage_Vlad_Makeev (second problem coordinator), netman (ex-second problem coordinator), kuviman (developer), fcspartakm (developer), SladeThe (developer) and gKseni (manager). Great contributions were provided by the students of the Saratov University: BledDest, awoo, vovuh, and Perforator — educational rounds, trainings in Codeforces::Gym and help in the development. I radiate the rays of gratitude to problem writers and testers: you create interesting content for many thousands of programmers from all over the world. Many thanks to all those who helped with the special projects, wrote interesting posts and good comments. All together, we were able to make Codeforces better!

So, let's move on to the results. It seems to me, this year again has something to brag about.

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 old handle would automatically redirect to the actual profile.

This year we have an improvement. If you took part in at least 10 rounds you can request handle of an inactive participant. It means that the participant should have a period of activity on Codeforces of at most 30 days, this period should be in 2012 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. If you can’t change handle to others, 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

Traditional magical tab has appeared in the profile setting. Happy New Year!

Hello everyone!

Another year has gone by. The last Codeforces contest of this year will be tomorrow, 29 Dec, 15:35 UTC.

The round details:

• combined div1+div2
• 8 problems
• 2.5 hours
• rated!!!

I'd like to thank the following people for helping with the round: KAN, winger, AlexFetisov, zemen, xiaowuc1, MikeMirzayanov. Without them, this round would not have been possible.

Scoring distribution will be posted later. Make sure to read ahead on the problems, since there may be some later problems that are easier for you. I hope to see you all at the contest, and good luck on the last chance to increase your rating this year!

EDIT1: The scoring distribution is 500-750-1000-1750-1750-2000-2750-3500

EDIT2: There will be a five minute delay for starting.

EDIT3: Editorial is here: http://codeforces.com/blog/entry/56713

EDIT4: Congratulations to the winners!

1. Petr
2. dotorya
3. V--o_o--V
4. Radewoosh
5. Endagorion
6. rng_58
7. PavelKunyavskiy
8. jqdai0815
9. mmaxio
10. Reyna

Hello Codeforces!

On December 28, 17:05 MSK Educational Codeforces Round 35 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 Div. 2. It will be held on extented ACM ICPC rules. After the end of the contest you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 7 problems and 2 hours to solve them.

The problems were prepared by Ivan BledDest Androsov, Vladimir vovuh Petrov, Alexey Perforator Ripinen and me.

Thanks Mike MikeMirzayanov Mirzayanov for great system Polygon!

Good luck to all participants!

We also have a message from our partners, Harbour.Space University:

Hello everyone!

Registration is open for our upcoming 1st Hello India x Russia Programming Bootcamp, with 20 teams signed up and counting, and we are looking forward to seeing you and your teammates in India this March!

More information

Register here

UPD: There will be 7 problems!

Congratulations to the winners:

Congratulations to the best hackers:

1761 successful hacks and 1237 unsuccessful hacks were made in total!

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

UPD2: Editorial is out

Codeforces Round #455 for Div 2 competitors will be held on December 27 at 19:35 MSK. As usual, Div 1 competitors can join out of competition.

The round will be rated.

This round is based on tasks for summer contest for interns algO(1). If you have seen the problems from that contest before, please don't participate in the round. The problems were prepared by Maxim Kalinin (slycelote), Alexander Milanin (Milanin), Ibragim Ismailov (ibra) and me (Nickolas).

The competitors will be given six problems and two hours to solve them. The scoring distribution will be 500-1000-1500-1750-2000-2500.

We hope you'll like the problems. Good luck!

UPD: thanks to HellKitsune, vintage_Vlad_Makeev, 300iq, Arpa and Livace for testing the problems!

UPD: The contest is over. Editorial can be found here.

Congratulations to winners!

Div. 2:

1. MegaOwIer
2. skuecrk
3. Vergara
4. Luqman
5. UoA_Kaori

Div. 1:

1. Benq
2. dotorya
3. uwi
4. I_love_Tanya_Romanova
5. ksun48