import java.util.*

fun inputList() = 
    readLine()!!.split(" ").map(String::toLong)

fun sum_digits(num_: Long): Long {
    var num = num_
    var rtn: Long = 0
    while(num != 0.toLong()) {
        val tmp = num % 10
        rtn += tmp
        num /= 10
    }
    return rtn
}

fun gcd(a: Long, b: Long): Long = 
    if(b != 0.toLong()) a else gcd(b, a % b)

fun lcm(a: Long, b: Long) = 
    if(b==0.toLong()) a else gcd(b, a % b)

fun main(args: Array<String>) {
    val (h, w) = inputList()
    val (h_, w_) = inputList()

    println((h-h_) * (w - w_))
}
