{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE BangPatterns #-}

import Control.Applicative
import Control.Monad
import Control.Arrow
import Data.List
import Data.Maybe
import Data.Char
import Text.Printf

readInt = ( readLn :: IO Int )
getInts = map ( read :: String -> Int ) . words <$> getLine

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

main = do
	s <- getLine
	let
		chars = filter ( not . ( `elem` s ) ) [ 'a' .. 'z' ]
		res = map ( : "" ) chars ++ [ "None" ]
	putStrLn $ head res
