921. Minimum Add to Make Parentheses Valid

#link to problem https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/description/
class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        stack = []
        for i in s:
            if not stack:
                stack.append(i)
            elif i == ')' and stack[-1] == '(':
                stack.pop()
            else:
                stack.append(i)
        return len(stack)