#include <algorithm>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <queue>
#include <stack>
#include <utility>
#include <vector>
using std::cerr;
using std::cin;
using std::cout;
using std::endl;

void OutputError(std::string s) {
  cerr << "\033[93m" << s << "\033[m" << endl;
  return;
}

int main(void) {
  cout << std::fixed << std::setprecision(10);
  cin.tie(0);
  std::ios::sync_with_stdio(false);
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int result = 0;
  for (int i = a; i <= b; i++) {
    if (i < c || d < i) {
      result += (d - c + 1);
    } else {
      result += (d - c);
    }
  }
  cout << result << endl;
  return 0;
}