function Main(input){

    var H = input.split("\n")[0].split(" ")[0];
    var W = input.split("\n")[0].split(" ")[1];
    var h = input.split("\n")[1].split(" ")[0];
    var w = input.split("\n")[1].split(" ")[1];
    H = parseInt(H,10);
    W = parseInt(W,10);
    h = parseInt(h,10);
    w = parseInt(w,10);

    var a = H * W - (H * w + W * h);
    var b = a + h * w;
    console.log(b);



}

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