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