#include<bits/stdc++.h>
using namespace std;
int f[29][29],num[29];
int main(){
	memset(f,0,sizeof(f));
	memset(num,0,sizeof(num));
	int x,cnt=0,ans=0;
	cin>>x;
	if(x==0){cout<<0;return 0;}
	for(int i=1;i<=x;i++){
		int a,b;
		cin>>a>>b;
		f[i][a]=1;
		f[i][b]=1;
		num[i]=i; 
	}
	do{
		for(int i=1;i<=x;i++)
			if(f[i][num[i]]==1)cnt++;
		if(cnt==x)ans++;
		cnt=0;
	}while(next_permutation(num+1,num+1+x));
	cout<<ans<<endl;
	return 0;
}