import qualified Data.Vector                   as V
import           Data.List
import           Control.Applicative
import           Control.Monad

main :: IO ()
main = do
  s <- getLine
  putStrLn $ solve s ['a' .. 'z']

solve :: String -> String -> String
solve s []       = "None"
solve s (c : cs) = if c `elem` s then solve s cs else [c]
