#include <bits/stdc++.h>
using namespace std;
#define int long long

int N;

signed main(){
  cin>>N;
  vector<pair<int,int>> A(N);
  for(int i = 0; i < N; i++){
  	cin>>A[i].first>>A[i].second;
  }
  vector<int> B(N);
  for(int i = 0; i < N; i++){
  	B[i] = i;
  }
  int ans = 0;
  do{
  	int now = 0;
  	bool flag = true;
  	for(int i = 0; i < N; i++){
  		if(A[B[i]].first > now){
  			now = A[B[i]].first;
  		}
  		else if(A[B[i]].second < now){
  			flag = false;
  		}
  	}
  	ans += flag;
  }while(next_permutation(B.begin(),B.end()));
  cout << ans << "\n";
}