#include <bits/stdc++.h>
using namespace std;
long long re() {
  long long x;
  scanf("%lld", &x);
  return x;
}
int main() {
  long long y1 = re(), x1 = re(), y2 = re(), x2 = re();
  long long h = (x2 - x1) / 2 + 1;
  cout << h * (((y2 - y1 + 1) + 1) / 2) + (h - 1) * ((y2 - y1 + 1) / 2) << endl;
}
