#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
constexpr ll mod = 1e9 + 7;
constexpr ll INF = (1LL << 62) - (1LL << 31) - 1;

#define REP(i, init, n) for(int i = (int)(init); i < (int)(n); i++)
#define RREP(i, init, n) for(int i = (int)(init); i >= (int)(n); i--)
#define All(A) A.begin(), A.end()
#define rAll(A) A.rbegin(), A.rend()

#define vi vector<int>
#define vl vector<long>
#define vvi vector<vector<int>>
#define vvl vector<vector<long>>
#define pint pair<int, int>
#define plong pair<long, long>

int N;
vector<pint> P;

void solve() {
    vi ord(N);
    iota(All(ord), 0);
    int ans = 0;
    do {
        bool ok = true;
        REP(i, 0, N - 1) {
            int pl = P[ord[i]].first;
            REP(j, i + 1, N) {
                int nr = P[ord[j]].second;
                if(nr < pl) ok = false;
            }
        }
        if(ok) {
            /*
            REP(i, 0, N) {
                cerr << P[ord[i]].first << " " << P[ord[i]].second << endl;
            }
            cerr << "-----" << endl;
            */
            ans++;
        }
    } while(next_permutation(All(ord)));
    cout << ans << endl;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    cin >> N;
    P.resize(N);
    REP(i, 0, N) cin >> P[i].first >> P[i].second;
    solve();
}