So.. Do you (all of you who says that input format is correct) mean that books are stored in a way that the last page is the leftmost one? If so than it's not surprising that the solution which swaps numbers of the books gets AC.

Hah, I wasn't familiar with this particular trick, but I've done enough of the "simple" problems on Timus now to know that many problems derive their difficulty not from the underlying algorithm, but in correctly understanding problem statements which are sometimes tricky, ambiguous, or rely on unstated assumptions (like this one, assuming how people "normally" put books on shelves, and which side of the book contains the first page).

So I'm learning to always ask "what's the catch?" and got AC first time on this one (after initial WA on many other "simple" problems).   So solving Timus problems is successfully training me to read problems very carefully!