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

int main(){
    ll n; cin >> n;
    vector<ll> l(n),r(n);
    for(ll i = 0;i<n;i++)cin >> l[i] >> r[i];
    vector<ll> p(n);
    iota(p.begin(),p.end(),0);
    ll ans = 0;
    do{
        bool f = true;
        vector<ll> x(n);
        x[0] = l[p[0]];
        for(ll i = 1;i<n;i++){
            x[i] = max(x[i-1],l[p[i]]);
            if(x[i]>r[p[i]])f = false;
        }
        if(f)ans++;
    }while(next_permutation(p.begin(),p.end()));
    cout << ans << endl;
}