Count Good RBS

A string $X$ of length $M$ (where $M\geq 2$) containing only opening and closing parentheses (the characters `'('` and `')'`) is called *good* if it satisfies the following conditions: - $X$ is a regular bracket sequence$^\dagger$. - $X[2, M-1]$ is also a regular bracket sequence. $X[l, r]$ denotes the substring of $X$ starting at index $l$ and ending at index $r$. For example, `()` and `(())`