#include <bits/stdc++.h>
//#include <atcoder/modint>

using namespace std;
//using namespace atcoder;
using ll = long long;
//using mint = modint998244353;

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int N, ans=0;
    cin >> N;
    vector<int> idx(N), L(N), R(N);
    for (int i=0; i<N; i++){
        cin >> L[i] >> R[i];
        idx[i] = i;
    }
    do{
        int now=0;
        bool ok=1;
        for (auto i : idx){
            if (R[i] < now) ok=0;
            now = max(L[i], now);
        }
        ans += ok;
    } while(next_permutation(idx.begin(), idx.end()));

    cout << ans << endl;

    return 0;
}