object Main extends App {
  val sc = new java.util.Scanner(System.in)
//  val l = List.fill(5)(sc.nextInt)
  val h, w = sc.nextInt
  val h2, w2 = sc.nextInt

  val ans = h * w - (h2 * w + w2 * h - h2 * w2)
  println(ans)
}