import Data.List

solve :: [(Char, Char)] -> String
solve [(c1, c2)] = [c2]
solve ((c1, c2):cs)
  | c1 > c2 = [c2]
  | otherwise = solve cs

main :: IO ()
main = do
  s <- nub . sort <$> getLine
  putStrLn $ if length s == 26 then "None" else solve (zip s ['a'..'z'])