// Accepted
// Time : 0.076

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#define N 51
using namespace std;

int ar[N],l=0;

int gcd (int a , int b)
{
    //int r=1;

    if (a>b)
    {
        swap(a,b);
    }

    int r=1;

    while (r!=0)
    {
        r = b%a;
        b = a;
        a = r;
    }

    return b;
}

int main()
{
    int cnt;

    int n;

    while (cin >> n)
    {
        if (n==0)
        {
            break;
        }

        for (int i=0;i<n;i++)
        {
            cin >> ar[i];
        }

        cnt = 0;
        int sonn = n*(n-1)/2; // Set of Natural Number = (n*(n-1))/2;

        for (int i=0;i<n-1;i++)
        {
            for (int j=i+1;j<n;j++)
            {
                if (gcd(ar[i],ar[j]) == 1)
                {
                    //printf("(%d %d) ",ar[i],ar[j]);
                    cnt++;
                }
            }
        }

        if (cnt == 0)
        {
            printf ("No estimate for this data set.\n");
        }

        else
        {
            printf("%.6lf\n",sqrt((1.0*6.0*sonn)/cnt));
        }
    }

    return 0;
}