#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define ALL(a) (a).begin(), (a).end()
#define Yes cout << "Yes" << endl
#define No cout << "No" << endl
#define endl '\n'

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;
    cin >> N;
    vector<pair<int, int>> P(N);
    for (int i = 0; i < N; i++) {
        int l, r;
        cin >> l >> r;
        P[i] = {l, r};
    }
    int ans = 0;
    sort(ALL(P));
    do {
        bool ok = true;
        int cur_dif = 0;
        for (int i = 0; i < N; i++) {
            ll need = max(P[i].first, cur_dif);
            if (need > P[i].second) {
                ok = false;
                break;
            }
            cur_dif = need;
        }
        if (ok)
            ans++;
    } while (next_permutation(ALL(P)));
    cout << ans << endl;
}