#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n;
  cin >> n;
  vector<pair<int, int>> v(n);
  rep(i, n) cin >> v[i].first >> v[i].second;
  int ans = 0;
  vector<int> p(n);
  iota(p.begin(), p.end(), 0);
  do {
    bool ok = true;
    int maxim = 0;
    rep(i, n - 1) {
      maxim = max(maxim, v[p[i]].first);
      if (maxim > v[p[i + 1]].second) ok = false;
    }
    if (ok) ans++;
  } while (next_permutation(p.begin(), p.end()));
  cout << ans << '\n';
  return 0;
}