const main = (input) => {
  input = input.split('\n');
  const HW = input[0].split(' ');
  const hw = input[1].split(' ');
  console.log(HW[0]*HW[1] - hw[0]*HW[1] - HW[0]*hw[1] + hw[0]*hw[1]);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));
