#ifndef ONLINE_JUDGE
#define _GLIBCXX_DEBUG //[]で配列外参照をするとエラーにしてくれる。上下のやつがないとTLEになるので注意 ABC311Eのサンプル4みたいなデバック中のTLEは防げないので注意
#endif
#include <bits/stdc++.h>
#include <algorithm>
#include <cmath> // M_PIを使用するため
using namespace std;

using ll = long long;
using ld = long double;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rrep(i, n) for (ll i = (ll)n - 1; i >= 0; --i)

const ll INF = (1LL << 62);
const ll null = -1LL;
template <typename T>
using vc = vector<T>; // prioriy_queueに必要なのでここにこれ書いてます
template <typename T>
using vv = vc<vc<T>>;
template <typename T>
using vvv = vv<vc<T>>;
using vl = vc<ll>;
using vvl = vv<ll>;
using vvvl = vv<vl>;
using vvvvl = vv<vvl>;
using vs = vc<string>;
using vvs = vv<string>;
using vb = vc<bool>;
using vvb = vc<vb>;
using P = pair<ll, ll>;

template <class T>
istream &operator>>(istream &i, vc<T> &v)
{
    rep(j, size(v)) i >> v[j];
    return i;
}
// それぞれ「下，上，右，左」に対応
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

#define nall(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()

template <class T>
bool chmin(T &a, T b)
{
    if (a > b)
    {
        a = b;
        return true;
    }
    return false;
}
template <class T>
bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}

#define pb push_back
#define eb emplace_back
#define em emplace
#define pob pop_back
#define next_p(v) next_permutation(v.begin(), v.end())

void solve()
{
    ll n; cin >> n;
    vl p(n); iota(nall(p),0);
    vl l(n), r(n);
    rep(i,n) cin >> l[i] >> r[i];
    ll ans = 0;

    do
    {
        /* code */
        bool isOK = true;
        ll left = 0;
        rep(i,n) {
            if (r[p[i]] < left) isOK = false;
            left = max(l[p[i]],left);
        }
        if (isOK) ++ans;
    } while (next_p(p));

    cout << ans << endl;
    
}

int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll testcases = 1ll;
    // cin >> testcases;
    rep(_, testcases) solve();

    return 0;
}