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