921

class Solution {
public:
    int minAddToMakeValid(string S) {
        int open, ans;
        open = ans = 0;
        for(char c: S){
            if(c == '(') open++;
            if(c == ')' && !open) ans++;
            if(c == ')' && open) open--;
        }
        ans += open;
        return ans;
    }
};