function Main(input) {
	input = input.trim();
  var H = parseInt(input.split("\n")[0].split(" ")[0]);
  var W = parseInt(input.split("\n")[0].split(" ")[1]);
  var h = parseInt(input.split("\n")[1].split(" ")[0]);
  var w = parseInt(input.split("\n")[1].split(" ")[1]);
  console.log((H - h) * (W - w));
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));
