#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0; i<(n); i++)
#define rep1(i,n) for(int i=1; i<=(n); i++)
#define sz(x) int(x.size())
#define all(x) (x).begin(),(x).end()
#define lINF ll(2e18)//LONG_LONG_MAX  //ll
#define iINF int(2e9+100)//INT_MAX //int
#define yes "Yes"
#define no "No"
#define kotae cout<<ans<<endl;
#define dame { puts("-1"); return 0;}
#define dame0 { puts("0"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
#define db cout<<'@'<<endl;
#define el cout<<'\n';
#define pc(x) __builtin_popcount(x)
#define pcl(x) __builtin_popcountll(x)
using ll=long long;
using ull=unsigned long long;
using Pii=pair<int,int>;
using Pil=pair<int,ll>;
using Pli=pair<ll,int>;
using Pll=pair<ll,ll>;
using Pci=pair<char,int>;
using Pcl=pair<char,ll>;
template <typename T>
using pqg = priority_queue<T,vector<T>,greater<T>>;
using vi=vector<int>;
using vi2=vector<vector<int>>;
using vi3=vector<vector<vector<int>>>;
using vl=vector<ll>;
using vl2=vector<vector<ll>>;
using vl3=vector<vector<vector<ll>>>;
using vs=vector<string>;
using vpii=vector<Pii>;
using vpil=vector<Pil>;
using vpci=vector<Pci>;
using vpcl=vector<Pcl>;
using Ti=tuple<int,int,int>;
using vti=vector<Ti>;
void coutdouble(double x) { printf("%.10f\n", x); }
void coutvi(vi vec) { for (int k : vec)cout << k << ' '; cout << endl; return; }
void coutvl(vl vec) { for (ll k : vec)cout << k << ' '; cout << endl; return; }
void coutseti(set<int> st) { for (int k : st)cout << k << ' '; cout << endl; return; }
void coutsetl(set<ll> st) { for (ll k : st)cout << k << ' '; cout << endl; return; }
void chmax(int &a, int b){ a = max(a, b); return;}
void chmin(int &a, int b){ a = min(a, b); return;}
void chmaxl(ll &a, ll b){ a = max(a, b); return;}
void chminl(ll &a, ll b){ a = min(a, b); return;}
void rev(vi &a){ reverse(all(a)); return;}
template <typename T>
void srt(T &a){ sort(all(a)); return;}
template <typename T>
void srtr(T &a){ sort(a.rbegin(),a.rend()); return;}


void solve();

int main() {  

  int t = 1;
  //cin >> t;
  rep(i,t) solve();

  return 0;
}
void solve() { 

  int n;
  cin >> n;
  vpii p(n);
  rep(i,n) cin >> p[i].first >> p[i].second;
  sort(all(p));
  int ans = 0;
  vi a(n);
  rep(i,n) a[i] = i;
  do{
    int now = 0;
    bool ok = true;
    rep(i,n){
      if(now > p[a[i]].second) ok = false;
      now = max(now, p[a[i]].first);
    }
    if(ok) ans++;
  }while(next_permutation(all(a)));
  kotae
  return;
}