function main(input) {
    const args = input.split('\n');
    var one = args[0].split(' ');
    var two = args[1].split(' ');

    var H = parseInt(one[0], 10);
    var W = parseInt(one[1], 10);

    var h = parseInt(two[0], 10);
    var w = parseInt(two[1], 10);

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


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