-- リジャッジ落ち修正
-- putStrLn -> putStr

-- Try yukicoder
-- author: Leonardone @ NEETSDKASU

import qualified Data.Map as Map

main = putStr . unlines . solve =<< getLine

hoge [] = ["Impossible"]
hoge xs = xs

solve xs = ans where
    xm = foldl f Map.empty xs
    f a x = Map.insertWith (+) x 1 a
    ans = hoge . map (return.fst) . filter h $ map g ['a' .. 'm']
    g c = (c, f xm c)
    h (c, m) = (13,14) == xx where
        xx = foldl u (0,0) ['a' .. 'm']
        u (a,b) x = case Map.lookup x m of
            Just k  -> (a+1,b+k)
            Nothing -> (0,0)