'use strict';
function Main(input) {
    input = input.split("\n");
    let tmp = input[0].split(" ");
    const H = parseInt(tmp[0]);
    const W = parseInt(tmp[1]);
    tmp = input[1].split(" ");
    const h = parseInt(tmp[0]);
    const w = parseInt(tmp[1]);

    console.log((H - h) * (W - w));

}

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