'use strict'

function Main(input){
	let temp = input.trim().split("\n");
	let temp2 = temp[0].split(" ");
	let H = parseInt(temp2[0]);
	let W = parseInt(temp2[1]);
	
	temp2 = temp[1].split(" ");
	let h = parseInt(temp2[0]);
	let w = parseInt(temp2[1]);

	let sum = H*W;

	sum = sum - ( h*W + H*w ) + h*w;
	
	console.log(sum);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));