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

int main(){
    int n;
    cin >> n;

    vector<int>per (n);
    vector<vector<int>>diff (n,vector<int>(2));
    for(int i=0;i<n;i++){
        per[i] = i;
        cin >> diff[i][0] >> diff[i][1];
    }

    int ans = 0;
    do{
        int mn = 0;
        bool che = true;

        for(auto x:per){
            if(mn > diff[x][1])che = false;
            else mn = max(mn,diff[x][0]);
        }

        if(che) ans++;

    }while(next_permutation(per.begin(),per.end()));


    cout << ans << endl;
    
}