function main(input) {
   input = input.trim();
  const a = input.split("\n");
  const b = a[0].split(" ").map(Number);
  const c = a[1].split(" ").map(Number);
	console.log((b[0]-c[0])*(b[1]-c[1]));
  
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));