Is it really so hard?
Just print any valid solution... :)

You are to "determine the erroneously copied number". Of course, if you cannot say for sure that the error was while processing this very number (and if one of two or several numbers may be erroneous you cannot say it) then it must be an unrecoverable error - you just cannot restore the initial sequence unambiguously. It is a common knowledge in coding theory, but... the jury thinks differ. So you should output any of these pseudo-erroneous numbers.

P.S. Alexander Klepinin, I hope that the Championship of Ural will be MUCH better then the last contest. Do you remember Chelyabinsk-2003 (Round 1)?

I mean, it is really obvious that when there are several possible places where the error could have been made, then the error IS UNRECOVERABLE, please! And if the problem author thinks elseway, he really must say so clearly.

You know what I love about USACO? The rules say: this is not a tricky-input/tricky-output contest. And indeed, the problem statement tend to be as exhaustive as possible, so your solution is evaluated instead of your ability to avoid all the traps (unless the traps are really serious and reveal bugs in your code).

Most contests are usually like that, while the E statement was not only incomplete, but also misleading.