function main(input){
  const args1=input.split('\n')[0];
  const args2=input.split('\n')[1];
  const H = parseInt(args1.split(" ")[0], 10);
  const W = parseInt(args1.split(" ")[1], 10);
  const h = parseInt(args2.split(" ")[0], 10);
  const w = parseInt(args2.split(" ")[1], 10);
  console.log((H-h)*(W-w));

}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));