-- Helper Function isHex
isHex :: Char -> Bool
isHex x | ((y >= '0') && (y <= '9')) || ((y >= 'a') && (y <= 'f')) = True
        | otherwise = False
            where y = toLower x

g :: String -> Int
g xs    | length [ f x | x <- xs, isHex x ] > 0 = maximum [ f x | x <- xs, isHex x ]
        | otherwise = -1