One way to do it is count all the closing and opening parenthesis for each parenthesis and sum up the minimum of opening and closing parenthesis for a given parenthesis.

Edit- Ohk so this case is only valid if the opening comes before the closing one so its not a good solution. It can be done via dp maybe not sure

Here is a similar problem

https://csacademy.com/contest/interview-archive/task/longest-parenthesised-substring/