#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
#include <queue>
using namespace std;

struct Func_print
{
    void is_true(bool yes) {
        cout << (yes ? "Yes" : "No") << '\n';
    };
    void print_v_str(vector<string> v) {
        for (auto s : v) {
            for (auto c : s) {
                cout << c << ' ';
            }
            cout << '\n';
        }
    }

    void print_vv(vector<vector<int>> vv) {
        for (auto v : vv) {
            for (auto d : v) {
                cout << d << ' ';
            }
            cout << '\n';
        }
    }
};
Func_print Out;//出力用

#define REP(i, n) for (int i = 0; i < (n); i++)
#define VEC(type, name, n) vector<type> name(n); REP(i, n) cin >> name.at(i)
#define VV(type, name, h, w) vector<vector<type>> name(h, vector<type>(w)); REP(i, h) REP(j, w) cin >> name.at(i).at(j)
#define ALL(iterable) begin(iterable), end(iterable)



int main() {
///////////////////////////////
//お約束
    ios::sync_with_stdio(false);//printfとcoutを混在しないように注意
    //doubleの桁数指定は以下を使用する
    //cout << fixed << setprecision() << y << endl; 
    cin.tie(nullptr);
////////////////////////////////

    int N; cin >> N;
    vector<pair<int, int>> lr;
    REP(i, N) {
        int l, r; cin >> l >> r;
        lr.emplace_back(l, r);
    }

    vector<int> index(N);
    REP(i, N) index[i] = i;

    int ans = 0;
    do{
        bool success = true;
        int score_now = 0;
        REP(i, N) {
            auto [l, r] = lr[index[i]];
            if (score_now > r) {
                success = false;
                break;
            }
            
            score_now = max(score_now, l);
        }

        if (success) {
            ans ++;
        }
    }while (next_permutation(ALL(index)));

    cout << ans << endl;
}