#include<bits/stdc++.h>
using namespace std;
using ll =long long;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    ll N;
    cin>>N;
    vector<array<ll,2>> P(N);
    for(int i=0;i<N;i++)cin>>P[i][0]>>P[i][1];
    vector<ll> Q(N);
    for(int q=0;q<N;q++)Q[q]=q;
    ll an=0;
    do{
        ll x=-1;
        bool OK=1;
        for(int i=0;i<N;i++){
            if(x>P[Q[i]][1]){
                OK=0;
                break;
            }
            x=max(x,P[Q[i]][0]);

        }
        if(OK)an++;
    }while(next_permutation(Q.begin(),Q.end()));
    cout<<an<<"\n";
}