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

using ll = long long;
using ld = long double;

//using mint = modint; //mint::set_mod(M);
//using mint = modint998244353;
//using mint = modint1000000007;

template<class T> using pq = priority_queue<T>; //大きい順
template<class T> using pq_g = priority_queue<T, vector<T>, greater<T>>; //小さい順

#define vec_unique(v) v.erase(unique(v.begin(), v.end()), v.end()) //重複削除
#define vec_iota(v) iota(v.begin(), v.end(), 0) //0, 1, 2, 3, ..., n - 1にセット
#define concat(a, b) a.insert(a.end(), b.begin(), b.end())
#define debug(x) cerr << #x << " = " << x << endl

int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};
//int dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
//int dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};

#define INF 2e18
#define INF2 2e9

int main() {
    int n;
    cin >> n;
    vector<int> l(n);
    vector<int> r(n);
    for(int i = 0; i < n; i++) {
        cin >> l[i] >> r[i];
    }

    vector<int> p(n);
    vec_iota(p);
    int ans = 0;
    do {
        bool flag = true;
        int left = l[p[0]];
        for(int i = 0; i < n - 1; i++) {
            if(r[p[i + 1]] < left) {
                flag = false;
                break;
            }
            left = max(left, l[p[i + 1]]);
        }
        if(flag) ans++;
    } while(next_permutation(p.begin(), p.end()));

    cout << ans << endl;
    //cout << fixed << setprecision(15) << << endl;
    return 0;
}