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

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

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

    vector<array<int, 2>> LR(N);
    rep(i, N)
    {
        cin >> LR[i][0] >> LR[i][1];
    }

    vector<int> order(N);
    iota(order.begin(), order.end(), 0);

    int ans = 0;
    do
    {
        int now = -1;
        bool ok = true;
        for (auto i : order)
        {
            auto [L, R] = LR[i];
            if (R < now)
            {
                ok = false;
                break;
            }

            now = max(now, L);
        }

        ans += ok;
    } while (next_permutation(order.begin(), order.end()));

    cout << ans << endl;

    return 0;
}