import Control.Applicative
import qualified Data.Set as S

alphabetset = S.fromList "abcdefghijklmnopqrstuvwxyz"
 
solver0 s = S.toList $ S.difference alphabetset $ S.fromList s

solver s
    |length s2 == 0 = "None"
    |otherwise = (minimum s2):[]
    where
        s2 = solver0 s

main :: IO ()
main = do
    s <- getLine
    putStrLn $ solver s

