UPD: I was wrong :(
UPD2: I was wrong, again.
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.
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. Usually, if you want to write the statement formally, there is only one way that's much more convenient to use that the others.