"use strict";
function Main(input) {
    let lines = input.split("\n");
    let Ln = lines[0].split(/\s/).map((v) => v - 0);
    let H = Ln[0], W = Ln[1];
    Ln = lines[1].split(/\s/).map((v) => v - 0);
    let h = Ln[0], w = Ln[1];
    let wh = new Array(W).fill(0).map(() => new Array(H).fill(0));
    for (let y = 0; y < h; y++) 
        for (let x = 0; x < W; x++) { wh[x][y] = 1; }
    for (let y = 0; y < H; y++) 
        for (let x = 0; x < w; x++) { wh[x][y] = 1; }
    let ans = 0;
    for (let y = 0; y < H; y++) 
        for (let x = 0; x < W; x++) { if (wh[x][y] == 0) ans += 1; }
    console.log(ans);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
