function main(input) {
    var inputs = input.split("\n");
    var HW = inputs.shift().split(" ");
    var H = HW.shift();
    var W = HW.shift();
    var hw = inputs.shift().split(" ");
    var h = hw.shift();
    var w = hw.shift();
    console.log((H - h) * (W - w));
}
main(require("fs").readFileSync("/dev/stdin", "utf8"));