#include <bits/stdc++.h>

using namespace std;

void fast_io() {
	ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
}

int main() {
	fast_io();
	int n;
	cin >> n;
	vector<int> l(n), r(n);
	for (int i = 0; i < n; i++) {
		cin >> l[i] >> r[i];
	}
	vector<int> p(n);
	iota(p.begin(), p.end(), 0);
	int ans = 0;
	do {
		int cur = 0;
		bool ok = true;
		for (int i : p) {
			if (cur > r[i]) {
				ok = false;
				break;
			}
			cur = max(cur, l[i]);
		}
		ans += ok;
	} while (next_permutation(p.begin(), p.end()));
	cout << ans << endl;
}