main :: IO ()
main = do
    a1 <- map toInt . words <$> getLine
    a2 <- map toInt . words <$> getLine
    print $ a1 !! 0 * a1 !! 1 - a2 !! 0 * a1 !! 1 - a2 !! 1 * (a1 !! 0 - a2 !! 0)

toInt :: String -> Int
toInt x = read x :: Int