function Main(input) {

  input = input.split("\n");
  rowcol = input[0].split(" ");
  fill = input[1].split(" ");
  h = rowcol[0];
  w = rowcol[1];
  fillH = fill[0];
  fillW = fill[1];
  if(h == fillH || w == fillW) {
    console.log(0);
  } else {
    wcel = h * w - (w * fillH + ((h - fillH) * fillW));
    console.log(wcel);
  }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));