#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll = long long;
using P = pair<int, int>;

int main(void){
  int n; cin >> n;
  vector<P> diff(n);
  vector<int> p(n);
  for(int i=0; i<n; i++){
    cin >> diff[i].first >> diff[i].second;
    p[i]=i;
  }
  int ans=0;
  do{
    bool ok=true;
    int now=0;
    for(int i=0; i<n; i++){
      auto [l, r]=diff[p[i]];
      if(r<now){
        ok=false; break;
      }
      else{
        now=max(now, l);
      }
    }
    if(ok) ans++;
  }while(next_permutation(p.begin(), p.end()));
  cout << ans << endl;
  return 0;
}