void main() {
    int[] tmp = readln.split.to!(int[]);
    int hw = tmp[0], ww = tmp[1];
    tmp = readln.split.to!(int[]);
    int hb = tmp[0], wb = tmp[1];
    writeln((hw - hb) * (ww - wb));
}

import std.stdio;
import std.string;
import std.array;
import std.conv;
import std.algorithm;
import std.range;
import std.math;
import std.container;
import std.typecons;