import java.util.*
fun main(args: Array<String>){
	val cin = Scanner(System.`in`)
    val h = cin.nextInt()
    val w = cin.nextInt()

    val h_d = cin.nextInt()
    val w_d = cin.nextInt()

    println(h*w - h_d*w - w_d*h + h_d*w_d)
    
    
}