#include <bits/stdc++.h>
using namespace std;
int main()
{
    int N;
    cin >> N;
    vector<int> L(N), R(N);
    for (int i = 0; i < N; ++i)
        cin >> L[i] >> R[i];
    int ans = 0;
    vector<int> p(N);
    iota(p.begin(), p.end(), 0);
    do
    {
        int point = 0;
        bool check = true;
        for (int i = 0; i < N; ++i)
        {
            if (R[p[i]] < point)
                check = false;
            point = max(point, L[p[i]]);
        }
        if (check)
            ans++;
    } while (next_permutation(p.begin(), p.end()));
    cout << ans << endl;
    return 0;
}