import qualified Data.Set as S                                                                                             
main :: IO ()                                                                                                              
main = do                                                                                                                  
  s <- S.fromList <$> getLine :: IO (S.Set Char)                                                                           
  let a = filter (\x -> fst x) . map (\x -> (,) ((not . S.member x) s) x) $ ['a'..'z']                                     
  putStrLn $ if null a                                                                                                     
             then "None"                                                                                                   
             else [snd . head $ a]