function Main(input) {
    input = input.split("\n");
    var input1 = input[0].split(" ");
    var H = parseInt(input1[0]);
    var W = parseInt(input1[1]);

    var input2 = input[1].split(" ");
    var h = parseInt(input2[0]);
    var w = parseInt(input2[1]);

    var b = h * W + w * H - h * w;
    // console.log("b:" + b);
    var ans = H * W - b;
    console.log(ans)
}

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