object Main extends App {
  val line1 = io.StdIn.readLine.split(' ')
  val line2 = io.StdIn.readLine.split(' ')

  val H = line1(0).toInt
  val W = line1(1).toInt

  val h = line2(0).toInt
  val w = line2(1).toInt

  val white = (H-h) * (W-w)

  println(white)
}
