{-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
import qualified Data.Set as S

main :: IO ()
main = do
  s <- getLine
  putStrLn . maybe "None" (:[]) $ solve s

solve :: String -> Maybe Char
solve s = if S.null diff then Nothing else Just $ S.findMin diff where
  diff = S.difference (S.fromList "abcdefghijklmnopqrstuvwxyz") (S.fromList s)
