fun main (args : Array<String>) {
	val (H, W) = readLine()!!.split(" ").map(String::toInt)
	val (h, w) = readLine()!!.split(" ").map(String::toInt)
	val Height : Int = H.toInt()
	val Width : Int = W.toInt()
	val hei : Int = h.toInt()
	val wid : Int = w.toInt()
	println("${(Height - hei) * (Width - wid)}")
}