import java.util.Scanner

object Main {
  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val H = Integer.parseInt(sc.next())
    val W = Integer.parseInt(sc.next())
    val h = Integer.parseInt(sc.next())
    val w = Integer.parseInt(sc.next())

    println(H * W - h * W - H * w + h * w)
  }
}