#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < int(n); i++)
#define FOR(i,a,b) for(ll i = a; i < (ll)(b); i++)
#define all(a) (a).begin(),(a).end()
using ll = long long;
using VI = vector<int>;
using P = pair<int,int>;
const long long INF = 1LL << 60;
const int DX[] = {1,0,-1,0};
const int DY[] = {0,1,0,-1};

int main(){
	cin.tie(nullptr);
	ios_base::sync_with_stdio(false);
	
	ll n;
	cin >> n;
	vector<ll> l(n),r(n);
	rep(i,n) cin >> l[i] >> r[i];
	
	vector<int> a(n);
	iota(all(a),0);
	ll ans = 0;
	do{
		vector<ll> b(n);
		b[0] = l[a[0]];
		ans++;
		for(int i = 1; i < n; i++){
			if(b[i-1] > r[a[i]]) {ans--; break;}
			b[i] = max(l[a[i]],b[i-1]);
		}
		
	}while(next_permutation(all(a)));
	cout << ans << endl;
	return 0;
}