#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long ans = ((x2 - x1) / 2) * ((y2 - y1 + 1) / 2) +
                  ((x2 - x1) / 2 + 1) * ((y2 - y1 + 2) / 2);
  cout << ans;
  return 0;
}
