module Main where

import Control.Applicative
import Control.Monad
import Data.List
import qualified Data.Set as S

main :: IO ()
main = do
    ss <- getLine
    let s = S.fromList ss
    let t = S.fromDistinctAscList ['a'..'z']
    case S.minView (S.difference t s) of
        Nothing      -> putStrLn "None"
        Just (c, _)  -> putStrLn [c]