The statement of the problem says: "In order to let them do it the program has to be copied to all the N computers". You should know that N is the quantity of all the computers INCLUDING the one which has the program already installed. So the program has to be copied not to N, but to (N-1) computers. I've just got WA because of it.

Thanks! I got the same issue.
It's quite not obvious from the problem statement.