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

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

    int N; cin >> N;
    vector<tuple<int,int,int>> LR(N);
    int idx = 0;
    for(auto &[l,r,p] : LR) cin >> l >> r,p = idx++;
    sort(LR.begin(),LR.end());

    int answer = 0;
    do{
        int low = 0;
        for(auto [l,r,ign] : LR){
            if(r < low){low = -1; break;}
            low = max(l,low);
        }
        answer += low != -1;
    }while(next_permutation(LR.begin(),LR.end()));
    cout << answer << endl;
}