#include <bits/stdc++.h>
using namespace std;
pair<int,int> a[100];
int vis[100];
int n,sum;
void dfs(int step){
	if(n == step + 1){
		sum++;
		return;		
	}
	if(!vis[a[step].first]){
		vis[a[step].first] = 1;
		dfs(step+1);
		vis[a[step].first] = 0;
	}
	if(!vis[a[step].second]){
		vis[a[step].second] = 1;
		dfs(step+1);
		vis[a[step].second] = 0;
	}
}
int main(){
	cin >> n;
	if(!n){
		cout << n;
		return 0;
	}
	for(int i = 1;i <= n;i++)
		cin >> a[i].first >> a[i].second;
	dfs(1);
	cout << sum;
	return 0;
}

样例输出为6，竟然全AC，这是怎么肥事？