import Control.Monad
import Control.Applicative
import Data.List
import Data.Array

strToInt s = (read :: String -> Int)

main = do

   s <- getLine
   let ss = sub (sort s)
   let xs = sort ("abcdefghijklmnopqrstuvwxyz" \\ ss)
   
   if (length xs) == 0 then putStrLn "None"
                       else putStrLn [(head xs)]

sub :: [Char] -> [Char]
sub [] = []
sub (x:[]) = [x]
sub (x:xs) = if (x == (head xs))
             then sub xs
             else ([x] ++ sub xs)
