function Main(input) {

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