module Main where

import Data.List
import Control.Monad

solve :: Int -> Int -> Int -> Int -> Int
solve h w h2 w2 = h * w - h2 * w - w2 * h + h2 * w2

main = do
    [h, w] <- fmap read . words <$> getLine
    [h2, w2] <- fmap read . words <$> getLine
    print $ solve h w h2 w2


