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