function Main(input) {

    input = input.split("\n");

    tmp0 = input[0].split(' ');
    tmp1 = input[1].split(' ');

    var H = tmp0[0];
    var W = tmp0[1];
    var h = tmp1[0];
    var w = tmp1[1];

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

}

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