#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
#define all(a) (a).begin(), (a).end()
#define pb push_back
#define fi first
#define se second
mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
const ll MOD1000000007 = 1000000007;
const ll MOD998244353 = 998244353;
const ll MOD[3] = {999727999, 1070777777, 1000000007};
const ll LINF = 1LL << 60LL;
const int IINF = (1 << 30) - 1;


void solve(){
    int N; cin >> N;
    vector<int> L(N), R(N); for(int i=0; i<N; i++) cin >> L[i] >> R[i];

    vector<int> P(N); for(int i=0; i<N; i++) P[i] = i;
    int ans = 0;
    do{
        bool ok = true;
        vector<int> A(N);
        A[0] = L[P[0]];
        for(int i=1; i<N; i++){
            if(A[i-1] <= R[P[i]]){
                A[i] = max(A[i-1], L[P[i]]);
            }else{
                ok = false;
                break;
            }
        }
        ans += ok;
    }while(next_permutation(P.begin(), P.end()));

    cout << ans << "\n";
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    int T=1;
    //cin >> T;
    while(T--) solve();
}