#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> L(N);
    vector<int> R(N);
    for (int i = 0; i < N; ++i) cin >> L[i] >> R[i];
    vector<int> v(N);
	for (int i = 0; i < N; ++i) v[i] = i;
	int ans = 0;
	do {
		bool f = true;
		int cur = L[v[0]];
		for (int i = 0; i < N - 1; ++i) {
			if (cur <= R[v[i + 1]]) {
				cur = max(cur, L[v[i + 1]]);
			} else {
				f = false;
			}
		}
		if (f) ans++;
	} while (next_permutation(v.begin(), v.end()));
	cout << ans << endl;
}