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