#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (ll)(n); i++)
#define oke cout << "Yes" << '\n';
#define dame cout << "No" << '\n';
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
using HaiI = vector<vector<int>>;
using Hai2 = vector<vector<ll>>;
using HaiB = vector<vector<bool>>;
using Hai3 = vector<vector<vector<ll>>>;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout << fixed << setprecision(15);
	ll N;
	cin>>N;
	vector<ll>L(N);
	vector<ll>R(N);
	vector<ll>S(N);
	rep(i,N){
		cin>>L[i]>>R[i];
		S[i]=i;
	}
	ll cut=0;
	do{
		ll mi=-1;
		bool aori=false;
		rep(i,N){
			if(mi <= R[S[i]]){
				mi=max(mi,L[S[i]]);
			}else{
				aori=true;
			}
		}
		if(!aori){
			cut++;
		}
	}while(next_permutation(all(S)));
	cout<<cut<<"\n";
}