main = do
    --a <- readLn
    [a, b] <- map read . words <$> getLine
    --s <- getLine
    -- putStrLn $ show $ solve a b
    putStrLn $ solve a b

solve :: Int -> Int -> String
solve a b = if even (a + b) then show (round (fromIntegral (a + b) / 2)) else "Impossible"
