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