function Main(input) {
  input = input.trim()
  input = input.split("\n");
  input[0] = input[0].split(" ").map(Number);
  input[1] = input[1].split(" ").map(Number);
  console.log((input[0][0]-input[1][0])*(input[0][1]-input[1][1]))
}Main(require("fs").readFileSync("/dev/stdin", "utf8"));