const main = stdin => {
  const input = stdin.trim().split('\n')

  const [H, W] = input[0].split(' ').map(x => parseInt(x))
  const [h, w] = input[1].split(' ').map(x => parseInt(x))

  console.log((H - h) * (W - w))
}
// 標準入力
main(require("fs").readFileSync("/dev/stdin", "utf8"))
