// @ts-check
'use strict';
/** @param {string} input */

function _slicedToArray(arr, i) { return _arrayWithHoles(arr) || _iterableToArrayLimit(arr, i) || _nonIterableRest(); }

function _nonIterableRest() { throw new TypeError("Invalid attempt to destructure non-iterable instance"); }

function _iterableToArrayLimit(arr, i) { var _arr = []; var _n = true; var _d = false; var _e = undefined; try { for (var _i = arr[Symbol.iterator](), _s; !(_n = (_s = _i.next()).done); _n = true) { _arr.push(_s.value); if (i && _arr.length === i) break; } } catch (err) { _d = true; _e = err; } finally { try { if (!_n && _i["return"] != null) _i["return"](); } finally { if (_d) throw _e; } } return _arr; }

function _arrayWithHoles(arr) { if (Array.isArray(arr)) return arr; }

function main(input) {
  var _input$split$map = input.split(/\s/).map(Number),
      _input$split$map2 = _slicedToArray(_input$split$map, 4),
      H = _input$split$map2[0],
      W = _input$split$map2[1],
      h = _input$split$map2[2],
      w = _input$split$map2[3];

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

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