data Paren = Open
            | Close
            deriving Show

toParenList :: String -> Maybe [Paren]
toParenList [] = Just []
toParenList (x:xs) = case x of
    '(' -> fmap (Open:) (toParenList xs)
    ')' -> fmap (Close:) (toParenList xs)
    _   -> Nothing

floors :: [Paren] -> [Int]
floors  = scanl (\acc y -> case y of
        Open -> acc + 1
        Close -> acc - 1
        ) 0

f3 :: String -> Maybe [Int]
f3 = toParenList >=> pure . floors

floorEnds :: String -> Maybe Int
floorEnds input2 = case f3 input2 of
    Just x -> Just $ last x
    Nothing -> Nothing

whenEntersBasement :: String -> Maybe Int
whenEntersBasement input2 = case f3 input2 of
    Just x -> elemIndex (-1)  x
    Nothing -> Nothing


main' = fromMaybe "Invalid input" . fmap show . whenEntersBasement <$> readFile "input.txt"