function Main(input){
  input = input.split("\n");
  inputStr = input[0].split(" ");
  var H = Number.parseInt(inputStr[0]);
  var W = Number.parseInt(inputStr[1]);
  inputStr = input[1].split(" ");
  var h = Number.parseInt(inputStr[0]);
  var w = Number.parseInt(inputStr[1]);
  
  //計算する
  console.log((H*W)-(h*W)-(H*w)+(h*w));
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));