import Data.List

solve :: String -> String
solve s = solve' s ['a'..'z']

solve' :: String -> String -> String
solve' s t
  | t == "" = "None"
  | otherwise = if elem x s then solve' s xs else [x]
    where
      x = head t
      xs = tail t

main = do
  s <- getLine
  putStrLn $ solve s
