#include <iostream>
#include <iomanip>//小数点出力用
//cout << fixed << setprecision(10) << ans;
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <unordered_map>
using ll = long long;
using namespace std;
#define modPHash (ll)((1LL<<61)-1)
#define modP (ll)998244353
bool chkrng0idx(int pos, int sup) { return (0 <= pos && pos < sup); }
int clk4(int num) { return (num - 2) * (num % 2); }
void yn(bool tf) { cout << (tf ? "Yes\n" : "No\n"); }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int N;
	cin >> N;
	vector<int>L(N), 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 ok = 1;
		for (int i = 0;i < N - 1;i++) {
			for (int j = i + 1;j < N;j++) {
				if (R[V[j]] < L[V[i]]) {
					ok = 0;
				}
			}
			
		}
		if (ok) {
			ans++;
		}
	} while (next_permutation(V.begin(), V.end()));
	cout << ans;
	return 0;
}