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