#include <bits/stdc++.h>

#include <atcoder/fenwicktree.hpp>
#include <atcoder/segtree.hpp>
#include <atcoder/modint.hpp>
#include <atcoder/dsu.hpp>
#include <atcoder/lazysegtree.hpp>

using namespace atcoder;
using namespace std;
using ll = long long;
using ull = unsigned long long;
template <class T>
using max_heap = priority_queue<T>;
template <class T>
using min_heap = priority_queue<T, vector<T>, greater<>>;
ll ll_min = numeric_limits<ll>::min();
ll ll_max = numeric_limits<ll>::max();
ll ALPHABET_N = 26;
using mint = modint998244353;
#define rep(i, n) for (ll i = (ll)0; i < (ll)n; i++)
#define rep_(i, k, n) for (ll i = (ll)k; i < (ll)n; i++)
#define all(a) a.begin(), a.end()

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	ll n;
	cin >> n;
	vector<ll> L(n), R(n);
	rep(i, n) cin >> L[i] >> R[i];
	vector<ll> ids(n);
	iota(all(ids), 0);
	ll ans = 0;
	do
	{
		bool ok = true;
		ll p = 0;
		rep(i, n)
		{
			ll idx = ids[i];
			if (R[idx] < p)
			{
				ok = false;
				break;
			}
			p = max(p, L[idx]);
		}
		if (ok)
			ans++;
	} while (next_permutation(all(ids)));
	cout << ans << endl;
	return 0;
}