main = do
    ps <- map (map read . words) . lines <$> getContents
    print $ (ps !! 0 !! 0 * ps !! 1 !! 1 + ps !! 0 !! 1 * ps !! 1 !! 0) / (ps !! 0 !! 0 + ps !! 1 !! 0)