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