虫洞

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
using namespace std ;
struct coor{
    int x, y;
}loc[30];
int b[30],ans,n;
bool cmp(coor a,coor b){
    if(a.y==b.y)return a.x<b.x;
    return a.y<b.y;
}
int f(int num,int d,int begin,int p){
    if(p==1 && d==begin && num!=1) return 1;
    if(p==0)
        if(loc[d].y == loc[d+1].y) return f(num+1,d+1,begin,1);
        else return 0;
    if(p==1) return f(num+1,b[d],begin,0);
}
bool check(){
    for(int i=1;i<=n;i++)
        if( f(1,i,i,1) == 1 )return 1 ;
    return 0;
}
void dfs(int x){
    if(x==n+1){if(check()==1)ans++;return ;}
    if(b[x]==0)
        for(int i=x+1;i<=n;i++)
            if(b[i]==0){
                b[i]=x;b[x]=i;
                dfs(x+1);
                b[i]=0;b[x]=0;
            }
    else dfs(x+1);
    return ;
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d%d",&loc[i].x,&loc[i].y);
    sort(loc+1,loc+1+n,cmp);
    dfs(1);
    cout<<ans<<endl;
    return 0;
}