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

void main() {
	auto data = readln.split;
	auto H = data[0].to!int, W = data[1].to!int;
	data = readln.split;
	auto h = data[0].to!int, w = data[1].to!int;
	
	writeln(h*W + w*H - h*w);
}