function Main(input){
	inputs = input.split('\n');
    H = Number(inputs[0].split(' ')[0]);
    W = Number(inputs[0].split(' ')[1]);
    h = Number(inputs[1].split(' ')[0]);
    w = Number(inputs[1].split(' ')[1]);
    ans = (H-h)*(W-w);
    console.log(ans);
} 
 
Main(require("fs").readFileSync("/dev/stdin","utf8"));