#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i< (n); ++i)
#define repi(i, a, b) for (int i = (a); i < (b); ++i)
#define all(x) (x).begin(), (x).end()
#define fore(i, a) for(auto &i:a)
using ll = long long;
#define DEBUG(x) cerr << #x << ": "; for (auto _ : x) cerr << _ << " "; cerr << endl;
int main() {
	int n;cin >> n;
	vector<int> L(n), R(n), order(n);
	rep(i, n)cin >> L[i] >> R[i];
	rep(i, n)order[i] = i;
	int ans = 0;
	do{
		bool flag = true;
		rep(i, n-1){
			if(L[order[i]] > R[order[i+1]])flag = false;
		}
		if(flag)ans++;
	}while(next_permutation(all(order)));
	cout << ans << endl;

}