{-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
import qualified Data.Set    as S
import           Debug.Trace
main :: IO ()
main = do
  s <- getLine
  putStrLn . maybe "None" (:[]) $ solve s

solve :: String -> Maybe Char
solve s = S.lookupMin $ S.difference (S.fromList "abcdefghijklmnopqrstuvwxyz") (S.fromList s)
