It was assumed a such solution. Firstly, if the lengths of strings were not equal, obviously the answer is -1. In the other case for all positions we search for all letters and try to make changes to the current letter. To make it quick, we will precalc a matrix Z[26][26], in which the сell (i, j) - is the minimum cost of change of i-th the letter of the alphabet to j-th. It can be done for example by Floyd algorithm. If at some position there is no possible letter, the answer is -1. In the other case, we should sum the answer for all positions and write the resulting string.

Thanks alot :)..

Char array is faster than the C++ string, so I suggest you to use it when you can.