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