import Data.Maybe
main :: IO ()
main = do
  s <- getLine
  let r = solve s ['a'..'z']
  putStrLn $ if isNothing r then "None" else [fromJust r]

solve :: String -> String -> Maybe Char
solve _ "" = Nothing
solve s (c:cs) = 
  let 
    f = filter (/=c) s
     in if length f == length s then Just c else solve f cs