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