#include <bits/stdc++.h>
using namespace std;
int main() {
  long long c, h, x1, y1, x2, y2, ans;
  while (scanf("%I64d%I64d%I64d%I64d", &x1, &y1, &x2, &y2) != EOF) {
    h = y2 - y1 + 1;
    c = x2 - x1 + 1;
    ans = h * c / 2;
    if (h % 2 == 1) ans++;
    printf("%I64d\n", ans);
  }
  return 0;
}
