{-# 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 ((c1,c2):s) = if c1 /= c2 then [c2] else f s

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

