main = do
  s <- getLine
  print $ solve2 s
solve2 = solve2' ['a' .. 'z']
solve2' [] _ = "None"
solve2' (x : xs) [] = [x]
solve2' (x : xs) ys = let ys' = filter (/= x) ys in if length ys == length ys' then [x] else solve2' xs ys'