Minimum Swaps to Make Strings Equal

Given two equal-length strings of only 'x' and 'y', compute the minimum number of swaps between the strings (swap s1[i] with s2[j]) to make them identical or return -1 if impossible. Key idea: count mismatches of type "s1='x', s2='y'" and "s1='y', s2='x'"; pair same-type mismatches (one swap per pair) and handle any two opposite leftovers with two swaps, otherwise it's impossible if total mismatches is odd.