object Main {
  def main(args: Array[String]): Unit = {
    val Seq(largeH, largeW) = io.StdIn.readLine().split(" ").map(_.toInt).toSeq
    val Seq(h, w) = io.StdIn.readLine().split(" ").map(_.toInt).toSeq

    println(largeH * largeW - h * largeW - w * largeH + h * w)
  }
}
