import qualified Data.Set as S

type Used = S.Set Int

main :: IO ()
main = do
  s <- map fromEnum <$> getLine
  let m = S.fromList s
  putStrLn $ solve m

solve :: Used -> String
solve s = go $ map fromEnum ['a'..'z']
  where
    go :: [Int] -> String
    go [] = "None"
    go (c:cs)
      | S.member c s = go cs
      | otherwise = [toEnum c :: Char]
