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

using ll = int64_t;

ll N;
vector<ll> L, R;

void input() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> N;
    L.resize(N), R.resize(N);
    for (ll i=0; i<N; ++i) cin >> L[i] >> R[i];
}

void solve() {
    ll ans = 0;
    vector<ll> p(N);
    for (ll i=0; i<N; ++i) p[i] = i;
    do {
        ll pre = 0;
        bool flg = true;
        for (ll i=0; i<N; ++i) {
            if (R[p[i]] < pre) {
                flg = false;
                break;
            }
            if (L[p[i]] > pre) {
                pre = L[p[i]];
            }
        }
        if (flg) ++ans;
    } while (next_permutation(p.begin(), p.end()));
    cout << ans << "\n";
}

int main() {
    input();
    solve();
    return 0;
}

/* 考察

*/