object Main extends App {
  val in = scala.io.Source.stdin.getLines.toSeq
  val ls = in.map(_.split(' ').map(_.toInt).toIndexedSeq)
  val H = ls(0)(0)
  val W = ls(0)(1)
  val h = ls(1)(0)
  val w = ls(1)(1)
  val ret = (H-h)*(W-w)
  println(ret)
}
