import qualified Data.Map.Strict as M

type Count = Int
type Counter a = M.Map a Count

addElement :: Ord a => a -> Counter a -> Counter a
addElement x = M.insertWith (+) x 1

buildCounter :: Ord a => [a] -> Counter a
buildCounter = foldr addElement M.empty

countElem :: Ord a => Counter a -> a -> Count
countElem ctr el = M.findWithDefault 0 el ctr

judge :: String -> Maybe Char
judge s = if not canBuildPairs then Nothing else Just $ head $ M.keys $ M.filter (== 1) charCtr
    where
        charCtr = buildCounter s
        countCtr = buildCounter $ M.elems charCtr
        canBuildPairs = countElem countCtr 2 == 6 && countElem countCtr 1 == 1

main :: IO ()
main = do
    s <- getLine
    case judge s of
        Just c -> putChar c
        Nothing -> putStr "Impossible"
    putChar '\n'