#include <bits/stdc++.h>
using namespace std;
using ll=long long;
int main(){
  ll n;cin>>n;
  vector<int> perm(n);
  for (ll i=0; i<n;i++){
    perm[i]=i;
  }
  ll ans=0;
  vector<ll> l(n),r(n);
  for (ll i=0;i<n;i++){
    cin>>l[i]>>r[i];
  }
  
  do {
    bool fl=true;
    ll mini=l[perm[0]];
  
    for (int i=1;i<=n-1;i++){
      if (r[perm[i]] < mini) {
        fl=false;
        break;
      }
      mini=max(mini,l[perm[i]]);
      
    }
    if (fl) ans++;
  } while (next_permutation(perm.begin(),perm.end()));
  cout<<ans<<endl;
}