Hello everyone can someone explain this to me:

In this problem https://mirror.codeforces.com/contest/1512/problem/C

I have to construct a palindrome with limited 0 and 1s

The problem doesn't state that if we have remaining a or b (remaining 0 or 1 to add) the solution must return -1

and also in the image I sent,there is one case(1) that allow a or b to be remaining (a>0 or b>0) in the end

My main queston is:How in the sixth case we should return -1 contrarly to the seventh case?

I cannot yet finish a whole contest when i have been programming since long ago.I turned just yet to problemsets in order to improve.Except the lack of skills or knowledge I also feel intimidated by some problems.I have read a competitive programming book but it did just teach me some basic stuff like dynamic programming orbacktrackng which i haven't yet used in any problemin this competitive world.I must get better if anyone got any advice or wants to co-code during sundays I am up for it!!