Hello everyone,
I see that many people complain about copied problems. Their claim is that authors weren't able to come up with some problems, and decided to copy them from other sources instead.

Although a contest shouldn't have already used problems, please convince yourself that no problemsetter would copy problems deliberately. The presence of some problems from the Internet is accidental. So, it's not correct to accuse the authors to "copy" the problems.

FAQ:

Q. The statement is completely the same, isn't it obvious that the problem was copied?
A. No. Proof: I invented 844C - Sorting by Subsequences and 1088D - Ehab and another another xor problem, with the same statement.

Q. Maybe you remembered the statement because you had already read it without solving the problem?
A. No. Proof: I invented 1501C - Going Home and arc130_d before the contest.

Q. How is it possible that no author / tester was able to find the "copied" problem by googling?
A. Challenge: find arc115_e on Google, using only the statement of 1591F - Non-equal Neighbours.