## 1247. 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.

Example 1:

Input: s1 = "xx", s2 = "yy" Output: 1 Explanation: Swap s1[0] and s2[1], s1 = "yx", s2 = "yx".

Example 2:

Input: s1 = "xy", s2 = "yx" Output: 2 Explanation: Swap s1[0] and s2[0], s1 = "yy", s2 = "xx". Swap s1[0] and s2[1], s1 = "xy", s2 = "xy". Note that you cannot swap s1[0] and s1[1] 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'`

.

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