#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=long double;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repd(i,n) for(ll i=n-1;i>=0;i--)
#define rep1(i,n) for(ll i=1;i<=n;i++)
#define rep_lt(i,j,k) for(ll i=j;i<k;i++) //[j,k)
void printd(ld x){cout<<fixed<<setprecision(16)<<x<<endl;}
void printd2(ld x,ld y){
  cout<<fixed<<setprecision(16)<<x<<' '<<y<<endl;
}
template<typename Vec>
void vcout(const Vec& vec){
  for(auto& v:vec) cout<<v<<' ';
  cout<<'\n';
}
template<typename Vec>
void vcout_val(const Vec& vec){
  for(auto& v:vec) cout<<v.val()<<' ';
  cout<<'\n';
}
template<typename Vec>
void vvcout(const Vec& vec){
  for(auto& v:vec){
    for(auto& k:v) cout<<k;
    cout<<'\n';
  }
}
template<typename Vec>
void vvcout_s(const Vec& vec){
  for(auto& v:vec){
    for(auto& k:v) cout<<k<<' ';
    cout<<'\n';
  }
}
template<typename Vec>
void vvcout_val(const Vec& vec){
  for(auto& v:vec){
    for(auto& k:v) cout<<k.val()<<' ';
    cout<<'\n';
  }
}
template<typename T,typename U>
void chmax(T& a,const U& b){
  if(a<b) a=b;
}
template<typename T,typename U>
void chmin(T& a,const U& b) {
  if(a>b) a=b;
}
struct FastIO{
  FastIO(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
  }
}fastio;
void YN(bool state){cout<<(state?"Yes\n":"No\n");}
void CY(bool state){if(state)cout<<"Yes"<<endl,exit(0);}
void CN(bool state){if(state)cout<<"No"<<endl,exit(0);}
void Cm1(bool state){if(state)cout<<-1<<endl,exit(0);}
const ll INF=4e18;
#define all(p) p.begin(),p.end()
#define rall(p) p.rbegin(),p.rend()
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353;
//using mint=modint1000000007;
using T=tuple<ll,ll,ll>;
using P=pair<ll,ll>;

//考察

int main(){
  ll n;cin>>n;
  vector<ll> l(n),r(n);
  rep(i,n) cin>>l[i]>>r[i];

  vector<ll> p(n);
  rep(i,n) p[i]=i;

  ll ans=0;
  do{
    ll mn=0;
    bool ok=true;
    rep(i,n){
      if(r[p[i]]<mn) ok=false;
      chmax(mn,l[p[i]]);
    }
    if(ok) ans++;
  } while(next_permutation(all(p)));
  cout<<ans<<endl;
  return 0;
}