Minimum Swaps to Make Strings Equal

You are given two strings s1 and s2 of equal length consisting of letters "x " and "y " only. Your task is to make these two strings equal to each other. You can swap any two characters that belong to different strings, which means: swap s1[i] and s2[j]. Return the minimum number of swaps required to make s1 and s2 equal, or return -1 if it is impossible to do so.