Apply Operations to Make Two Strings Equal — Python

# Time:  O(n)
# Space: O(1)

# dp
class Solution(object):
    def minOperations(self, s1, s2, x):
        """
        :type s1: str
        :type s2: str
        :type x: int
        :rtype: int
        """
        parity = curr = prev = 0
        j = -1
        for i in xrange(len(s1)):
            if s1[i] == s2[i]:
                continue
            curr, prev = min(curr+x, prev+(i-j)*2 if j != -1 else float("inf")), curr
            j = i
            parity ^= 1
        return curr//2 if parity == 0 else -1