import Control.Monad

main = do
    [h1, w1] <- map read <$> words <$> getLine
    [h2, w2] <- map read <$> words <$> getLine
    putStrLn $ show $ solve h1 w1 h2 w2

solve :: Int -> Int -> Int -> Int -> Int
solve h1 w1 h2 w2 = (h1 - h2) * (w1 - w2)
