#include <bits/stdc++.h>
using namespace std;
// #include <atcoder/modint>
// using namespace atcoder;
// using mint = modint998244353;
using ll = long long;
#define fix(x) fixed << setprecision(x)
#define rep(i, n) for(int i = 0; i < n; ++i)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> l(n), r(n);
    rep(i,n) cin >> l[i] >> r[i];
    vector<int> p(n);
    iota(all(p),0);
    int ans = 0;
    do{
        int mi = 0;
        bool f = true;
        rep(i,n){
            if(mi>r[p[i]]){
                f = false;
                break;
            }
            chmax(mi,l[p[i]]);
        }
        ans += f;
    }while(next_permutation(all(p)));
    cout << ans << '\n';
    return 0;
}