nesting :: String -> Int
nesting s
  | null s    = 1
  | otherwise = go s []
  where
    go [] stack = if null stack then 1 else 0
    go (c : cs) stack
      | c == ')'  = pop cs stack
      | otherwise = go cs (c : stack)

    pop cs (top : rest)
      | top == '(' = go cs rest
    pop _ _ = 0