class Solution:

    def minimumSwap(self, s1: str, s2: str) -> int:

        count = {"xy":0,"yx":0}

        for i in range(len(s1)):

            if s1[i] == s2[i]:

                continue

            elif s1[i] == "x" and s2[i] == "y":

                count["xy"] += 1

            elif s1[i] == "y" and s2[i] == "x":

                count["yx"] += 1

        m = count["xy"] % 2

        n = count["yx"] % 2

        if [m,n] in [[1,0],[0,1]]:

            return -1

        else:

            return count["xy"]//2 + count["yx"]//2 + m + n 

class Solution:

    def minimumSwap(self, s1: str, s2: str) -> int:

        if len(s1) != len(s2): return -1

        xy = yx = 0

        for c1, c2 in zip(s1, s2):

            if c1 == c2: continue

            if c1 == 'x': xy += 1

            else: yx += 1

        if xy&1 != yx&1: return -1

        return ((xy+1)>>1) + ((yx+1)>>1)