I have been trying to solve Day1 C problem and there is no editorial(at least I didnt find it). I've tried understanding codes, but I have no idea. If somebody did solve it could you explain your idea?

Hello codeforces, Please, give information about Codeforces accounts of EJOI 2021 participants from your country.

I can not understand this part( If you could explain it, I would be very grateful^_^

UPD: I got it now

Do you know any information?(place/date)