"use strict";
const main = input => {
    let H = parseInt(input[0].split(' ')[0], 10);
    let W = parseInt(input[0].split(' ')[1], 10);
    let h = parseInt(input[1].split(' ')[0], 10);
    let w = parseInt(input[1].split(' ')[1], 10);
    console.log((H-h)*(W-w));
    
}
main(require("fs").readFileSync("/dev/stdin", "utf8").split('\n'));