import Data.Maybe

maybeRead :: Read a => String -> Maybe a
maybeRead s = case reads s of
                  [(x,"")]    -> Just x
                  _           -> Nothing

getListFromString :: String -> Maybe [Integer]
getListFromString str = maybeRead $ "[" ++ str ++ "]"

main :: IO ()
main = do
  putStrLn "Enter a list of numbers (separated by comma):"
  input <- getLine
  let maybeList = getListFromString input in
      case maybeList of
          Just l  -> print (sum l)
          Nothing -> putStrLn "Bad format. Good Bye."