import Data.List

solve :: [(Char, Char)] -> String
solve [] = "None"
solve ((c1, c2):cs)
  | c1 == '-' = [c2]
  | c1 > c2 = [c2]
  | otherwise = solve cs

main :: IO ()
main = do
  s <- take 26 . (++ replicate 26 '-') . nub . sort <$> getLine
  putStrLn . solve $ zip s ['a'..'z']