1247 - Minimum Swaps to Make Strings Equal\.

Medium

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.

Example 1:

Input: s1 = "xx", s2 = "yy"

Output: 1

Explanation: Swap s10 and s21, s1 = "yx", s2 = "yx".

Example 2:

Input: s1 = "xy", s2 = "yx"

Output: 2

Explanation:

Swap s10 and s20, s1 = "yy", s2 = "xx".

Swap s10 and s21, s1 = "xy", s2 = "xy".

Note that you cannot swap s10 and s11 to make s1 equal to "yx", cause we can only swap chars in different strings.

Example 3:

Input: s1 = "xx", s2 = "xy"

Output: -1

Constraints:

• 1 <= s1.length, s2.length <= 1000

• s1, s2 only contain 'x' or 'y'.