import Data.List

main :: IO ()
main = do
  s <- getLine
  putStrLn $ none s

none
  :: String -> String
none ls = helper ls' ['a' .. 'z']
  where
    ls' = nub . sort $ ls
    helper 
      :: String -> String -> String
    helper []     [] = "None"
    helper (l:ls) (x:xs)
      | l == x    = helper ls xs
      | otherwise = [x]