type Height = Int
type Width = Int
type Area = Int

area :: (Height, Width) -> Area
area (height, width) = do
  height * width

main = do
  [height, width] <- map read . words <$> getLine :: IO [Int]
  [targetHeight, targetWidth] <- map read . words <$> getLine :: IO [Int]
  let wholeArea  = area (height, width)
  let targetArea = area (targetHeight, targetWidth)
  print $ wholeArea - targetArea
  