object Main{
  def main(args: Array[String]){
    if (sys.env.getOrElse("TEST", "")=="1"){
 
    }else{
      val input=io.Source.stdin.getLines().mkString("\n")
      println(solve(input).toString.trim());
    }
  }
 
 
  def solve(input:String):Int ={
    val bigs = input.split("\n")(0).split(" ").map(_ .toInt)
    val mins = input.split("\n")(1).split(" ").map(_ .toInt)
    ( bigs(0) - mins(0) ) * (bigs(1) - mins(1))
  }
}