import Data.Char as Char

solve :: String -> String
solve = solveInner 0

solveInner :: Int -> String -> String
solveInner idx s
    | idx >= 26 = "None"
    | elem c s = solveInner (idx + 1) s
    | otherwise = [c]
    where
        c = chr (idx + (Char.ord 'a'))

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