Happy new year, Codeforces!
We are glad to invite you to take part in Hello 2026, which will start on Jan/07/2026 17:35 (Moscow time). You will be given 9 problems and 3 hours to solve them. At least one problem will be interactive, so please make sure to read the guide for interactive problems before the contest. This round is open and rated for everyone.
Problems are authored by wangmarui, Gold14526, Hoks_, Neil_Qian, Network_Error, jiazhichen844 and xvchongyv.
We would like to thank:
- Um_nik for pre-reviewing the round.
- Proof_by_QED for his wonderful coordination.
- Alexdat2000 for Russian translation.
- Geothermal, StarSilk, and Dominater069 for nutella testing.
- Mindeveloped, ETO_leader, nldr, max0810, A_G, _istil, ian_HLX, nifeshe, paulzrm, sammyuri, catgirl and Intellegent for red testing.
- cuteSubtask, LucaLucaM, cry, Genius_Star_rgw, Friedrich, incra, lkrqwq, LaDeX, pichax, shiny_shine, efishel, Edeeva, Register, AksLolCoding, Watersphere, Lilypad, tzl_Dedicatus545, Caylex and waauto for yellow testing.
- NATO, Cfz_qwq, gotexans, gza, wuhudsm, madlogic and Heartquake for purple testing.
- tybbs, larush, pianapolataa, Terminator-XII, hollis_yang, Nukumizu-Anna, shengxuanyi, chromate00 and cby_ for blue testing.
- wizardMarshall, Solwek, sztcodeforces and lottle1212 for cyan testing.
- yaotianhao0906, lovelyfish and YSY_ for green testing.
- chenzhishuo2012 and Lvl200GoldenDragon for gray testing.
- Lynette_D for unrated testing.
- MikeMirzayanov and KAN for the great Codeforces and Polygon platforms.
- You for participating in this round.
Score Distribution: $$$500 - 1000 - 1500 - (1000 - 2000) - 2250 - 2750 - 4000 - 4000 - (4500 - 1000)$$$
Good Luck & Have Fun!
Update 1: The score distribution have been changed a bit. In particular, problem I will be split into two subtasks.
Update 2: Hacks will be disabled on problems A-D.
Update 3: editorial — https://codeforces.me/blog/entry/149944
Update 4: Congratulations to the winners!
Auto comment: topic has been updated by wangmarui (previous revision, new revision, compare).
As a "tester," I have yet to test the round
UPDATE: I tested the round, and the problems are very interesting!
As a tester, I can confirm that the problems and cats are cute.
.
As a tester, I hope everyone can enjoy the first contest of the year.
Hello 2026!
As a writer, I think that problems are interesting, have fun!
I just want to know why this comment received so many downvotes. Did the author make any mistakes in the problem setting of this contest?
I'm a little worried about this contest. It would be better if someone could explain why.
Bad Reputation
thank you for telling
This contest is good, but this author's reputation may not good...
as a nobody, what exactly did the big pretender do?
Bad reputation, deliberate downvotes from some foolish friends (?), and a bunch of bandwagoners.
I don't know why my friends would make such false statements, it makes me disappointing.
Maybe this comment will also has many downvotes from my friends.
Please stop discussing this matter,have fun in the contest, and enjoy the problems, thanks. :)
Actually, there's no one behind you at all.
'woah have i smashed up my noip my friends? i've only passed a single problem...'
100+92+68+60=320.
the pretenders are divided into four levels: normal pretenders, mad pretenders, ultimate pretenders and jiazhichen pretender.
I am 100 points away from the CQ line...
320-217=103.
nope, it should be 'i am yawa stniop 001 from the CQ line.'
Someone please explain,, what happened? Who is he? please
Well, jiazhichen844 is a famous Chinese P words speaker. That means he always trys to hide his ability when he talks to other while he is actually a legendary grandmaster, it makes us very uncomfortable and confused so that many friends dislike him. And more importantly, he hardly ever take a shower. I think you should remember this name, because almost every Chinese OIer knows him and always talk about him and his P words.
It seems that you hadn't taken any shower for a long time. I guess that's why your smell is strong. You should take a shower now. And speak less P words so that you will not get so many downvotes. My coach hzg06 had ever told me not to play with P words speaker.
Maybe you're right; he does need to take a shower. Just like this:
It might be my first contest, hope to get good grades. Anyway, best wishes everyone!
As a writer, I was amazed to discover that only 9 of the nine problems are truly excellent.
As a tester, I think the problems are all quite interesting, and hope everyone can get good experience.
I hope I can solve from A to D1!
as a friend of the writers (including the infamous pretender jiazhichen844), please upvote the announcement!
as a friend of a friend of the writers, please upvote the announcement!
Wow Chinese round!!!
emm
wow jiazhichen
This has to be like the most tasks i have ever seen in the contest.
I had an opportunity to participate in testing the round, but I would prefer to participate in this round...
Well actually I've seen my friends in tester list many times, but they haven't called me to join even once.
Looks like some friends are only friends on good days , but participating in the contest is better anyway.
Chinese round but not Chinese-friendly time...
Why is the start round $$$22$$$:$$$35$$$? Shouldn't it be $$$21$$$:$$$35^{UTC+7}$$$?
UPD: It's fixed
As a tester, I hope everyone can achieve high scores in this contest.
Wait. I tested the round?
Note the unusual starting time?
they forgor about timezones
Oh thanks
There's "unrated testing" now?
What, where?
As a tester, I hope everyone enjoys the new year.
Hi everyone! I'm new to Codeforces, hoping to pass 3 problems. Hope everyone can get good grades~
Maybe on this contest, I can solve 5-6 problems and get rating of 1600.
When does the Christmas theme of snow go away? When I use black mode, it just looks really ugly because of the snowflakes.
Use F12, go to dev tools, styles, and uncheck the box with the image source.
(https://limewire.com/d/yt47k#ybzCeorUov)
Click Download All
Extract the files into a folder.
Load into Chrome
dannyrios
black mode...
Hope to become expert again.
As a tester, you can't Register the round by clicking me.
but you can Register by clicking me ;)
As a tester, I can confirm that the problems and wangmarui are both cute.
no both for >2 things.
Fortunately, it's not an English grammar contest.
Hello 2026!
As a writer, I hope this contest will make everyone happy!
waitinggggg
conflict with codechef?
yeah, before 2020, this used to a major thing, but codechef isn't attracting top CPers so nobody is interested in even shifting it
Hello 2026 on a weekday? :(
its conflicting with CodeChef starters contest, i wish we could do something about it?
I forgot what is my current password and the password saved in Google password manager is showing wrong when putting there for changing my handle. I wanna change my handle name but couldn't because of current password. Can anyone help ?
Use the "Forgot password" option on Codeforces and reset it via email.
New year, new me. I'll become an Expert this year and possibly a Candidate Master.
I trust you
As a tester, I hope everyone can enjoy the first contest of the year.
And happy new year to all of you!
Do you know if this contest is going to be div 2 or in the middle of div 2-3 like the goodbye 2025 one?
It will be similar to GoodBye 2025 one (to be more precise, a Div1 + Div2 level). To get an even better iota, refer to past Hello and GoodBye Rounds; in addition to Global, Pinely and CodeTon rounds.
In short, it is going to be at a tougher level than a standard Div2 Round (as whole).
Excited for Hello 2026! Thanks to all the problem setters, testers, and coordinators for putting together such a great round. Can’t wait to try all 9 problems and see who cracks the interactive one first!
Good luck to everyone participating!
D2 seems a tough nut to crack (might require a drastic optimisation in comparison to D1). Based on the score distribution, it seems feasible to solve E before D (as whole) or D2 (at least).
As a tester, I hope everyone can enjoy the first contest of the year.Wishing everyone a wonderful start to the new year!
As a writer,hope you enjoy the contest HL,2026
As a tester, hope you can enjoy the contest. GOODBYE 2025, and HELLO 2026!
As a tester,the problems are interesting,I hope all of you can enjoy these problems.
Happy New Year!
LucaLucaM please farm contribution in the comments.
As a tester, the only thing I know is that armani stronger with you is better than prada paradigme
When should I start studying harder problems? I've solved roughly 80 800's. I've only done a few problems that were harder from contests. I had to upsolve two of them. I want to get stronger at my Data Structures skills and technical ability.
read this
Just here to say I'm the one who made the 600 upvotes 601
hope to have a great contest
Hope that B not harder than C, and C don't harder than D(please).
I'm looking forward for this contest, First 2026 contest!!
My rating is 2020 now, hoping for a +6 to hit 2026 :))
Hoping to reach expert after this contest :3
If this contest is having an interactive problem at problem C then this is going to be an interesting round
It's a shame that I have no time to participate in this contest.
But hope this will be a good beginning of 2026!
No one care about clashing with Codechef contest ?
No
will this contest be fair?
Auto comment: topic has been updated by wangmarui (previous revision, new revision, compare).
4 minutes left for extra reg, but can't register.
UPD: idk, what happened on while registering or I misunderstood something, but it got fixed.
As a nobody, is it rated?
good bless whoever created the questions,not even one can be solved by me
How 2.1k people solved C but only 1k has solved D
Im registered for the contest however cant submit any code and it tells me that i need to be registered to submit?
How can i fix this>
At least one problem will be interactiveI can't find any Interactive problem in this contest
its problem G
Thanks!
An1kv is cheater that texted abt it in tg group (i can proof it), hope he ll be banned
Thank you for contest, very interesting C and quite funny realization in D! (Awww, I solve A and B purely by intuition....)
Huge gap between F and G. How to solve (or rationally guess) G and H?
Well in G you kinda want to keep as many snakes alive as possible or kill as many as possible, otherwise you get a lot of uncertainty in any harder cases, and it turns out the shortest possible queries ("L" and "R") give you a lot of info. You can stresstest 3-tuples of queries that either move snakes very far in some direction or by at most 1 in any direction, checking if the number of solutions that match the answers for a pre-chosen solution is always 1, to test this assumption.
D2 shouldn't have been in the contest
And why so ?
95% of the problem's difficulty comes from it's messy implementation
I think a contest should have some kind of implementation part, and considering my solution, I think D2's implementation is quite short
That probably means you overcomplicated it. My solution: go from bottom to top, color bottom layer using colors 1 to its size, then for each layer, sort the vertices by the smallest color of any children (leaves go in the back), give the first one highest color possible, then give the others smallest remaining colors in order (so 1, 2, 3.. until you reach the color of the first vertex, then that+1 etc.). This works, because there is only one vertex with a child of that color (there's only on child of that color in the layer below), and due to the sorting and choosing highest possible color for the first vertex, any color you give to the other vertices had to be a child of an earlier vertex, or not used at all. Implementation is rather easy, probably about the same level as other D-s.
How to solve Problem C (seems to be a binary search problem)
yup, i did binary search
I used two pointers
What's your logic could you pls explain
Just a simple greedy is enough,if you goes to $$$[k - x,k + y]$$$ you need $$$x + y + \max(x,y) - 1$$$.
check my submission
did we split a hard D into "guess in D1 and prove in D2" or what?
There are some constructions of the coloring that TLE on D2, but basically yeah.
No, you can remain guessing in D2. Just write any greedy algorithm which will WA on #2, and then add a little randomness.
In fact, if we model it as a perfect matching in a bipartite graph, we will obtain a rather dense graph, and its solution set is very large. So this makes sense.
Shouldn't greedy algorithms with randomness probably fail on trees where almost every layer has exactly 3 vertices (except the root, obviously)? There's enough different situations there that just about any greedy that isn't guaranteed to be correct can fail there, yet it's constrained enough that a failing situation is not that unlikely to occur.
it was more like, guess d1 then guess d2
For problem C, for input 7 7 4 , how to get 6 bases fortified ?
I can't reach 6, I can reach till 5 only.
Do nothing on day 1. On start of day 2 you have 2 soldiers on base 4. On days 2 and 3 move the 2 soldiers to bases 3 and 2.
Now there is 1 soldier on base 2, 3, and 2 on 4. Wait 1 more day. Then on the next 3 days move to bases 5,6,7.
Aaahhh, I thought we could only move all the soldiers in a base to the next or previous base at once.
Wait 2 turns, use 3 moves to move 3 soldiers right, leaving them in bases one by one, them use 2 moves to move 2 soldiers left and levae them in bases one by one.
0) 0 0 0 1 0 0 0 (initially)
1) 0 0 1 1 0 0 0
2) 0 1 0 2 0 0 0
3) 1 0 0 3 0 0 0
4) 1 0 0 1 3 0 0
5) 1 0 0 2 1 2 0
6) 1 0 0 3 1 1 1
7) 1 0 3 1 1 1 1
in day 4 to day 5. how are you moving partially ? We have to move either all solders or move none at all.
Read the problem again, and you will notice something.
I personally think that this is a trap.
You give out an order by choosing a base i (1≤i≤n), and any number of soldiers inside that base (which is allowed to be 0 or all soldiers in that base currently.
We can move either 0 or all soldiers i that base. we can't move partial.
It says any number of soldiers, and "any" is in bold...
it also says, " 0 or all the soldiers in that base currently".
So yeah, problem statement should have been, "which is allowed to be between 0 and all soldiers in that base currently".
You can't say, two contradicting things in same statement. Or wait you can say two contradicting things in same statement. Its confusing. No, its not confusing.
The exact statement was “…any number of soldiers inside that base (which is allowed to be 0 or all soldiers in that base currently). Then, tell all soldiers you ordered to either…”
It’s not saying two contradicting things. It’s just emphasizing that you can also choose 0 or all soldiers, in case anyone wonder if "any soldiers" would include that or not. Although I'm not sure who would
for first 2 days let soldier accumulate now you have 3 soldiers after day 2 now shift those 3 to left 3 blocks that would take 3 more days so now days count is 5 now meanwhile soldiers are accumulating at home postion so after 5 days soldiers at home position are 3 but days remaining is 2 so move those 2 to right on 6th day and move one of them to next right position on day 7. Sorry for this messy explanation, but I hope it clears your doubt.
in first 5 days you can send 3 to left/right, and keep 2 on base camp. then start sending opossite side. finally in 7 days, 2+3+1=6
Is 2183C - War Strategy inspired by generals.io?
First thing that came to mind after reading the statement. XD
yeah I'm a big fan of it so I wrote it based off of that
QAQ Why this hard QAQAQAQAQAQ
How does D2 have 1,300 solves?
as an expert, Im not able to implement the MEX finding algorithm correctly LLLLOOOOOLLLLLLL
I spent whole time in solving D2 and get wrong answer on it.
A guessForces contest is not the best way to start 2026.
people hate:
1)guessforces
2)speedforces
3)mathforces
What else?
queueforces
Unratedforces
Cloudfareforces
AIableButRatedForces
As a contestant, I found the problems very interesting. I personally loved it
Will be there post with solutions?
https://codeforces.me/blog/entry/149944
Thank you!
C too hard
again 100% system test cause of bug submission
Sorry, but why it happens again? link
Hello, An1kv cheated today and I can proof it (He texted about it in tg group chat)
Auto comment: topic has been updated by wangmarui (previous revision, new revision, compare).
Can anyone explain me how 6 is answer for this testcase n=7 m=7 k=4 for question 3 I tried but not able to get more then 5
It was my first contest. Someone close to me was terminally ill, and solving problems was the only way I knew to stay steady. Between wrong answers and small victories, I found a little peace. Thank you for this platform.
when rating will be given?
Hi everyone, I've written two detailed comments, specifically on solving C & D1 in a little different way than what was intended as per the editorial: -
I hope these help! :)
Anyone with a nice constant-time solution for C? I had one (but small mistake), just curious how to solve it simply/elegantly. Thanks :)
Don't get the point of this round being put under the "Hello 2026" tag.
A,B ok.
C: A good problem, but it looks like a copy-paste of generals.io and the smallest time to capture a specific set of tiles is exactly generals.io's game-opening strategy (wait until you have a number of army and then make the other tiles yours).
D1: ok.
D2: I know the authors probably put D2 because contestants might guess the lower bound of the answer in D1. But it made the problem too difficult for this position. (I can say for sure that Not only E but also F<D1).
E: ok problem, but it's a bit boring if you've seen this formula before(?). So it's too easy a problem.
F: just open the square and it's brainless.
So it's clear that E and F can't be put in that place. I'd say E 1800, F 2000? if F is put in E position then obviously many more will pass it.
G/H/I1/I2: I didn't think carefully, but tourist didn't solve any of them. Too hard. They together with E,F made the big gap. Seen from the ranklist, the round failed to separate CM~IGM contestants with the number of problems they solved — only the penalty.
So it has bad differentiation, and its A~F is not good either.
I'd say bad round.
Agreed. (But I didn't solve the brainless F in contest.)
G and H are interesting but too hard.
Yes, I had an idea with Ferrers diagrams: Explanation on Ferrers diagrams can be found here for partitions but I didn´t get too far into solving. Maybe something I could do this weekend.
Was B really so easy? I hate myself.
Yes Sal Volcano
Because of this contest, I got 1000+ rating!
Problems A through D require no algorithm, only guesses and observations. If you cannot guess them, you will be jailed for 3 hours.
i didn't guess A! (i thought it was
count(1)>=count(0))How is the answer for testcase 6 of problem D2 valid?
Input:
Output:
In the 1st subset don't
5and10have the same distance fromnode 1?5 is in the 3rd subset. The first number on each line is subset size — think about the kind of annoying parsing you'd have to do in the checker if that information wasn't given directly on the output.
my second contest! did pretty well :)
as a friend of a friend of the writers, please upvote the announcement!
11
as a friend of a friend of a friend of the writers, please upvote the announcement!
Did he just do the 67 joke...
Nice Contest! Got to learn new stuff about trees.
Hello, I acknowledge the similarity warning related to my submission for Problem 2813A – Binary Array Game from the Hello 2026 contest.I understand that a significant coincidence between solutions has been detected and that such similarity, even when unintentional, is considered a violation of Codeforces contest rules. I would like to state that there was no intention to copy or misuse any solution. I realize that unintentional leakage can still lead to such coincidences, and I sincerely apologize for this mistake. I fully understand the importance of maintaining contest integrity and fully assure you that this will not happen again in any future contests.
Regarding the similarity warning for Problem 2813D – Tree Coloring (Easy): I would like to clarify that I did not copy or share my solution. I wrote the code entirely on my own using VS Code locally. Given the nature of the problem, I believe the similarity is purely coincidental and likely stems from the standard logic required to solve it. I fully respect the contest rules and integrity.
My friend took my code and submitted and now I have faced an violation of copied code. I don't have any conclusive evidence and I don't know what to do as an action for this.
Hello, Regarding my submission 356826690 for problem 2183D1, and my recent submissions in consecutive contests, I would like to clarify the situation. During two consecutive contests, I made a serious mistake by discussing the general approach of problems with a friend during the contest and allowing him to use my internet hotspot. Later, I realized that he submitted solutions that significantly coincided with mine. I now fully understand that even discussing approaches during an ongoing contest is considered solution leakage and a clear violation of the contest rules. I sincerely apologize for this behavior. I did not use AI tools or copy any solution myself, but I acknowledge that my actions led to unintentional solution leakage, and repeating this mistake was entirely my responsibility. This was my first experience with such violations, and I assure that this will not happen again. I will strictly follow all Codeforces rules in future contests. Thank you for your time and understanding.
-Solution 356800081 for the problem 2183A I would like to clarify that I did not cheat during this round.
The similarity in solutions appears to be coincidental and due to the use of a common source or standard algorithmic approach that was publicly available and published before the competition. This material was accessible to all participants and was not created during the round.
I did not communicate with any other participants during the contest and solved the problems independently based on my prior knowledge.
I am happy to provide further clarification of my solution approach if needed.