#include <iostream.h>
#include <stdio.h>
#include <fstream.h>
#include <math.h>
ifstream f("pi.in");
#define cin f
int fun(int m,int n)
{
int q;
q=m%n;
while(q!=0)
{
m=n;
n=q;
q=m%n;
}
return n;
}


int main()
{
    int n,i,j;
    int a[51];
    while(cin>>n)
    {
        int sum=0;
        if (n==0) return 0;
        for(i=0;i<n;i++)
        cin>>a[i];
        for(i=0;i<n;i++)
        for(j=i+1;j<n;j++)
        {
            int x=a[i];
            int y=a[j];
            if (fun(x,y)==1) sum++;
        }
        if(sum==0)
        cout<<"No estimate for this data set.\n";
        else
        printf("%0.6f\n",sqrt(6*n*(n-1)/2/sum));
    }
} 

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