function Main(input) {
  input = input.split("\n");
  var H = parseInt(input[0].split(" ")[0]);
  var W = parseInt(input[0].split(" ")[1]);
  var h = parseInt(input[1].split(" ")[0]);
  var w = parseInt(input[1].split(" ")[1]);

  var AtCoder = function(){};

  AtCoder.prototype = {
    procedure: function(){
      return H*W - (h*W+w*H-h*w);
    }
  }
  var atcoder = new AtCoder();
  console.log(atcoder.procedure());
}

var input=Main(require("fs").readFileSync("/dev/stdin", "utf8"));
// var input=`5 5
// 2 3`
// Main(input);
