'use strict';
function main(input) {
  input = input.split('\n');

  let row1 = input[0].split(' ');
  let row2 = input[1].split(' ');
  
  let H = row1[0];
  let W = row1[1];
  let h = row2[0];
  let w = row2[1];

  let total = H * W;

  // console.log(H, W, h, w);
  console.log(total - (H * w + (W - w) * h));
}

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