#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, n) for (int i = a; i < (int)(n); i++)
using ll = long long;

int main()
{
    int n;
    cin >> n;
    vector<int> l(n), r(n);
    rep(i, 0, n) cin >> l[i] >> r[i];
    vector<int> p(n);
    iota(p.begin(), p.end(), 0);
    int ans = 0;
    do
    {
        int pre_diff = l[p[0]];
        bool ok = true;
        rep(i, 1, n)
        {
            if (r[p[i]] < pre_diff)
            {
                ok = false;
                break;
            }
            else if (i != n - 1)
            {
                pre_diff = max(pre_diff, l[p[i]]);
            }
        }
        ans += ok;
    } while (next_permutation(p.begin(), p.end()));

    cout << ans << endl;
}