//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define all(a) (a).begin(), (a).end()
using ll = long long;
const ll INF32 = 2e9;
const ll INF64 = 4e18;

void printYN(bool ok){
    if(ok)cout << "Yes" << endl;
    else cout << "No" << endl;
    return;
}

int main() {
    int N;
    cin >> N;
    vector<int> L(N), R(N);
    rep(i, N)cin >> L[i] >> R[i];
    vector<int> P(N);
    rep(i, N)P[i] = i;
    int cnt = 0;
    do {
        bool ok = true;
        int dif = L[P[0]];
        for(int i = 1; i < N; i++){
            if(dif>R[P[i]])ok = false;
            dif = max(dif, L[P[i]]);
        }
        if(ok)cnt++;
    }while(next_permutation(all(P)));
    cout << cnt << endl;
    return 0;
}