#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, n) for(int i=(0+s); i < (n+s); i++)

//pair<T, T>-> P<T>
template<class T> using P = pair<T, T>;
template<class T> struct item {T A, B, C;};

//secondとfirstが交わらないかチェック
vector<vector<ll>> canditates;
vector<bool> visited;
int N;
void dfs(int depth, vector<ll> p){
    if(depth==N){
        canditates.push_back(p);
        return;
    }
    for(int i=0; i<N; i++){
        if(visited[i])continue;
        p.push_back(i);
        visited[i]=true;
        dfs(depth+1, p);
        p.pop_back();
        visited[i]=false;
    }
    return;
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> N;
    visited.resize(N, false);
    vector<P<ll>> myvec(N);
    rep(i, 0, N) cin >> myvec[i].first >> myvec[i].second;

    dfs(0, {});
    ll cnt=0;
    for(vector<ll> p: canditates){
        bool ok=true;
        ll min_value=0;  //最低限超えていてほしい値
        for(int i=0; i<N-1; i++)
        {
            if(myvec[p[i]].first>myvec[p[i+1]].second || myvec[p[i+1]].second<min_value){ok=false;break;}
            min_value=max(min_value, myvec[p[i]].first);
        }
        if(ok){
            cnt++;
            // for(ll a : p)cout << a;
            // cout << endl;
        }
    }
    cout << cnt << endl;
    return 0;
}