#include <bits/stdc++.h>
using namespace std;

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  vector<int> V(101);
  for (int i = a; i <= b; i++) V.at(i)++;
  int cnt = 0;
  for (int i = c; i <= d; i++) if (V.at(i)) cnt++;
  cout << (b - a + 1) * (d - c + 1) - cnt << "\n";
}