Minimum Add to Make Parentheses Valid — Python

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

class Solution(object):
    def minAddToMakeValid(self, S):
        """
        :type S: str
        :rtype: int
        """
        add, bal, = 0, 0
        for c in S:
            bal += 1 if c == '(' else -1
            if bal == -1:
                add += 1
                bal += 1
        return add + bal