const main = (input) => {
  const inputs = input.split('\n');
  const H = Number.parseInt(inputs[0].split(" ")[0]);
  const W = Number.parseInt(inputs[0].split(" ")[1]);
  const h = Number.parseInt(inputs[1].split(" ")[0]);
  const w = Number.parseInt(inputs[1].split(" ")[1]);

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


}

main(require('fs').readFileSync('/dev/stdin', 'UTF-8'));
