#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (ll i = s; i < (ll)(t); i++)
#define all(x) begin(x), end(x)

template <class T> bool chmin(T& x, T y) {
	return x > y ? (x = y, true) : false;
}
template <class T> bool chmax(T& x, T y) {
	return x < y ? (x = y, true) : false;
}

struct io_setup {
	io_setup() {
		ios::sync_with_stdio(false);
		cin.tie(nullptr);
		cout << fixed << setprecision(15);
	}
} io_setup;

void solve() {
	int n;
	cin >> n;
	vector<int> l(n), r(n);
	rep(i, 0, n) cin >> l[i] >> r[i];
	vector<int> p(n);
	iota(p.begin(), p.end(), 0);
	int ans = 0;
	do {
		int x = -1;
		for (int i : p) {
			chmax(x, l[i]);
			if (x > r[i]) {
				x = -1;
				break;
			}
		}
		if (x != -1) ans++;
	} while (next_permutation(p.begin(), p.end()));
	cout << ans << "\n";
}

int main() {
	int t = 1;
	// cin >> t;
	while (t--) solve();
}