#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(false); cin.tie(0);
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  cout << (b - a + 1) * (d - c + 1) - max(0, min(b, d) - max(a, c) + 1) << endl;
  return 0;	
}