The Southwestern Europe Regional Contest will take place on the 23rd of April. It is the ICPC regional contest (i.e., the winning teams will advance to the ICPC World Finals) for teams from France, Israel, Italy, Portugal, Spain, and Switzerland.

The mirror contest SWERC 2021-2022 - Online Mirror (Unrated, ICPC Rules, Teams Preferred) will be held on Codeforces at Apr/24/2022 14:05 (Moscow time) and will last 5 hours.

The mirror contest will contain all the problems from the official competition plus some additional problems.

I am the chief judge for the competition and I want to thank:

- The amazing set of judges who proposed and prepared the problems: cescmentation_folch, cip999, Delfad0r, gangsterveggies, Giove, gog.gerard, majk, Petr, Simon, tap_tapii.
- The MIT team composed of Rewinding, TLE, antontrygubO_o for testing the round and Philae for proofreading the statements.
- Everyone involved in the organization of SWERC, in particular Dariost (director), edomora97 (technical director), Gianpaolo Agosta (director) and wil93 (secretary).
- MikeMirzayanov for Polygon (that we used to prepare the problems) and for letting us host the mirror on Codeforces.

I invite you to participate in the contest and I hope that you will like the problems.

On the difficulty

The contest features problems with **difficulties from div2A to div1F**; so anyone can find something at their level.

Many teams without much experience participate in SWERC, so the problem set should be enjoyable also for div2 contestants. On the other hand, solving all the problems should be challenging even for the strongest teams in the world: the MIT team did not AK in 5 hours.

**On the beauty**

Rules

- The contest is unrated, so your codeforces rating will not be affected.
- The scoring is ICPC-style: teams are first sorted by number of problems solved, then the time-penalty is used as a tie-break. An incorrect submission gives a 20 minutes penalty.
- We encourage participation as a team.
- If you are participating in a team, we encourage you to use only one computer for coding the solutions (as in an ICPC contest). Regarding using templates and copy-pasting code: feel free to do it.

**Rationale of rule 4.**

UPDATE: We hope you liked the problems, here is the editorial for all the problems in the mirror: https://mirror.codeforces.com/blog/entry/102042 .

UPDATE: Congratulatins to the all the participants of the onsite contest and in particular to the two gold medal winning teams, both solving 10 problems (and with a very similar penalty time):

**Raw Pots**--**Harbour.Space University****TAU++**--**Tel Aviv University**

And congratulations also to the four teams who managed to solve all the problems in the mirror:

orz (i wonder why no one else has commented this so far)

Why the registration is closed at once?

Edit: Now it's open. Thanks!

Is the online mirror private for certain users?

It is open for everyone.

When will the registration phase start?

Now it is open!

why some participants are colored red for this contest??

They registered twice (probably because they registered on their own, then another team registered them). Just unregister one of which and it should be fine.

It won't prevent them from joining the contest either way, but last I heard, if they submit, a random entity out of the two will get counted for submission.

clash with google codejam 1B

By 5 minutes??

yes :)

I don't think it matters much for 5 hr long contest :)

Yes, we are aware of it. We think that the last 5 minutes of a 5 hours mirror and/or the first 5 minutes of google code jam round 1b are not super important and it's acceptable if they overlap.

nice

You are absolutely right, this contest is completely useless for you.

Good chance for training on online contest because rated will not be affected

I am excited to participate with my friends today in this contest.

Is there a live scoreboard of the official competition?

UPD: Found it: https://scoreboard.swerc.eu

Rule 5. If you want to participate "competitively" in the mirror, refrain from opening the scoreboard so that you don't obtain any information on the difficulty of the problems.

Problem N be like :

Contestants with age 2,250,000 :

Hi, I'm new to this type of contests and I have trouble trying to register: https://streamable.com/jjl827 This was happening since I tried registering from before the contest started. Can I have some advice on what I can do for future contests please?

feeder1 needs to go to https://mirror.codeforces.com/contestRegistrants/1662/friends/true and unregister the team first. Just hit the red X. Then you're good to go

worked, thanks!

good round, really enjoy this round

thank you for the problem

problem D H is cool and O is also nice (i guess the traditional way works)

M is also a good (not sure because of not implement)

how solve to B C E F G J K L N?

M is very straightforward, just put maxR 'R' and maxW 'W'. how to solve D? My thought is remove all occurence of AA, BB, CC and replace AB with BA, CB with BC, then check if the two string is same. However it fails. What did I miss?

After AB->BA replaces some new occurrences of AA or BB may appear. But if you will repeat these 2 sets of substitutions until none could be applied any more, then, I guess, this will be correct.

I did it recursively. Let me wait for editorial or failure cases.

That should work. In a simpler O(N) constructions: basically send all the B's to the start (and remove BB), and use a stack to pair up AA and CC. I don't know how to prove it's correct though.

K:

Note that due to continuity and such arguments, it is not hard to verify that the sum of lengths can be made the same for all three triangles when you reach the minimum $$$r$$$. Let the given points be $$$A, B, C$$$. Construct points $$$A'_i, B'_i, C'_i$$$ with $$$i \in \{1, 2\}$$$ such that $$$BCA'_i$$$ etc. are all equilateral. Let's call a triangle

goodif the minimum sum is achieved at a vertex, andbadotherwise. If we choose an optimal residence point $$$D$$$, then there are at least two triangles among $$$DBC, DCA, DAB$$$ which are both good or both bad, since a triangle is good iff one angle of the triangle is $$$> 120^\circ$$$. In the case they are good, say $$$DAB$$$ and $$$DCA$$$ are good, we just need to ternary search for the best point on the corresponding perpendicular bisector of $$$BC$$$ (do this for all permutations to find the answer in this case). In the case they are bad, say $$$DAB$$$ and $$$DCA$$$ are bad, note that we have $$$DB' = DC'$$$ where $$$B', C'$$$ are on the opposite sides of $$$CA, AB$$$ as $$$B$$$ and $$$C$$$. So we should ternary search for the answer on the perpendicular bisector of $$$B'C'$$$ (and do it for $$$A'B'$$$ and $$$C'A'$$$ as well).Note that whenever we said that we need to do a ternary search, there are some more constraints to be satisfied. We note that the problem is to find a global minimum, and since the function is convex, we should just evaluate the answer for each case naively while ternary searching (without paying attention to such constraints). For instance, rather than using $$$DA'$$$, we can use $$$\max(DA', DA' ')$$$.

Now that the contest is over, why are we not able to submit to the problems in practice mode?

Is there any editorial?

Here it is! https://mirror.codeforces.com/blog/entry/102042

..

Problem I is a much easier version of USACO21DEC Silver Closest Cow Wins. In this problem $$$p_i$$$ is fixed and $$$N=1$$$.

Why still can't view the submissions?????

Should be fixed now.

thx!

will teams' pics be added when clicking on them on the scoreboard?

Yes, in the next few days the swerc.eu website will be updated with a copy of the scoreboard that includes team's photos.

When will we know who's invited to WF (now that all european regionals finished)?

MikeMirzayanov, it has been more than 2 weeks since this contest, please update problem ratings. Thanks.

I just came here to say how amazingly clever problem N is

Thank you :)