You are given two strings s1 and s2 of equal length, consisting only of the characters 'x' and 'y'.

In one operation, you can choose an index from s1 and an index from s2, then swap the characters at those positions.

Return the minimum number of swaps required to make both strings equal. If it is impossible to make them equal, return $-1$.

Constraints:

• $1 \leq$ s1.length, s2.length $\leq 1000$

• s1.length $=$ s2.length

• s1 and s2 consist only of 'x' and 'y'.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.