#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n;
	cin >> n;
	vector<int> ps(n), l(n), r(n);
	for(int i = 0; i < n; i++)
	{
		ps[i] = i;
		cin >> l[i] >> r[i];
	}
	int sun = 0;
	do{
		int bef = -998244353;
	  	bool ok = true;
	  	for(int v:ps)
	  	{
	  		if(bef > r[v]) ok = false;
	  		bef = max(bef, l[v]);
	  	}
	  	sun+=ok;
	}while(next_permutation(ps.begin(), ps.end()));
	cout << sun;
}