"use strict";

function Main(input) {
    input = input.split("\n");

    input[0] = input[0].split(" ");
    const maxH = parseInt(input[0][0], 10);
    const maxW = parseInt(input[0][1], 10);

    input[1] = input[1].split(" ");
    const h = parseInt(input[1][0], 10);
    const w = parseInt(input[1][1], 10);

    let answer = maxH * maxW;
    for (let i = 0; i < maxH; i++) {
        for (let j = 0; j < maxW; j++) {
            if (i < h || j < w) {
                answer--;
            }
        }
    }

    console.log(answer);
}

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