Leetcode 921

class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        invalid_lefts = 0
        invalid_rights = 0

        for char in s:
            if char == '(':
                invalid_lefts += 1
            elif char == ')':
                if invalid_lefts:
                    invalid_lefts -= 1
                else:
                    invalid_rights +=1

        return invalid_rights + invalid_lefts