#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  x1 += 1e9 + 1;
  y1 += 1e9 + 1;
  x2 += 1e9 + 1;
  y2 += 1e9 + 1;
  long long x = abs(x1 - x2) + 1, y = abs(y1 - y2) + 1;
  long long x_odd = x / 2, x_even = x / 2, y_odd = y / 2, y_even = y / 2;
  if (x & 1) x_odd++;
  if (y & 1) y_odd++;
  cout << x_odd * y_odd + x_even * y_even << endl;
}
