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