#include<iostream>

#include<cstdio>

#include<cstring>

#include<algorithm>

#include<cmath>

#define Maxn 1010

#define Maxm Maxn*Maxn

#define inf 1e16

#define eps 1e-6

using namespace std;

int vi[Maxn],n;

double dis[Maxn][Maxn],cost[Maxn][Maxn],benefit[Maxn][Maxn],far[Maxn];

struct Point{

    double x,y,z;

}p[Maxn];

void init()

{

    memset(dis,,sizeof(dis));

    memset(vi,,sizeof(vi));

    memset(cost,,sizeof(cost));

    memset(far,,sizeof(far));

}

double Dis(Point a,Point b)

{

    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));

}

double prime(double r)

{

    int i,j,temp;

    double ans,Max;

    memset(vi,,sizeof(vi));

    ans=;

    for(i=;i<=n;i++)

        far[i]=inf;

    far[]=;

    for(i=;i<=n;i++)

    {

        Max=inf;

        for(j=;j<=n;j++)

        {

            if(!vi[j]&&far[j]<Max)

            {

                Max=far[j];

                temp=j;

            }

        }

        vi[temp]=;

        ans+=Max;

       // dis[temp][j]-cost[temp][j]

        for(j=;j<=n;j++)

        {

            if(!vi[j]&&far[j]>cost[temp][j]-r*dis[temp][j])

                far[j]=cost[temp][j]-r*dis[temp][j];

        }

    }

    return ans;

}

int main()

{

    int i,j,a,b,c;

    double Max;

    while(scanf("%d",&n)!=EOF,n)

    {

        init();

        Max=-inf;

        for(i=;i<=n;i++)

        {

            scanf("%lf%lf%lf",&p[i].x,&p[i].y,&p[i].z);

        }

        for(i=;i<n;i++)

        {

            for(j=i+;j<=n;j++)

            {

                dis[j][i]=dis[i][j]=Dis(p[i],p[j]);

                Max=max(dis[i][j],Max);

                cost[i][j]=cost[j][i]=fabs(p[i].z-p[j].z);

            }

        }

        double l,r,mid;

        l=,r=;

        double temp;

        while(r-l>eps)

        {

            mid=(l+r)/;

            temp=prime(mid);

            if(temp>)

                l=mid;

            else

                r=mid;

        }

        printf("%.3lf\n",l);

    }

    return ;

}