import qualified Data.Map.Lazy as M

createStringMap :: String -> M.Map Char Int
createStringMap = foldr (\c acc -> M.insertWith (\n o -> o + 1) c 1 acc) M.empty

process :: String -> Maybe Char
process input
  | M.size stringMap == 7 && M.size singleMap == 1 = Just $ (M.keys singleMap) !! 0
  | otherwise = Nothing
  where stringMap = createStringMap input
        singleMap = M.filter (\c -> c == 1) stringMap

main :: IO ()
main = do
  input <- getLine
  case process input of
    Just x -> putStrLn [x]
    Nothing -> putStrLn "Impossible"