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