{-# LANGUAGE FlexibleContexts, OverloadedStrings #-}
import Control.Applicative
import Control.Monad
import qualified Data.ByteString.Char8 as B
import Data.Maybe (fromJust)
import Text.Printf
import Debug.Trace

f [] [] = "None"
f [] (c:_) = [c]
f (c1:s1) (c2:s2) = if c1 /= c2 then [c2] else f s1 s2

main = do
  s <- map B.head . B.group . B.sort <$> B.getLine
  putStrLn $ f s "abcdefghijklmnopqrstuvwxyz"
