import qualified Data.ByteString.Char8 as C8

main :: IO ()
main = do
  [h, w] <- readInts
  [a, b] <- readInts
  print $ (h - a) * (w - b)

readInts :: IO [Int]
readInts = map (fst . fromJust . C8.readInt) . C8.words <$> C8.getLine