#include<iostream>

 #include<cmath>

 #include<iomanip>

 #define MAXN 50

 using namespace std;

 int _m[MAXN];

 int gcd(int a,int b);

 int main()

 {

     //freopen("acm.acm","r",stdin);

     int n;

     int i;

     int j;

     double sum_all;

     double sum;

     while(cin>>n,n)

     {

         sum = ;

         sum_all = ;

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

         {

             cin>>_m[i];

         }

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

         {

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

             {

                 if(gcd(_m[i],_m[j]) == )

                 {

                     ++ sum;

                 }

                 ++ sum_all;

             }

         }

         if(sum == )

         {

             cout<<"No estimate for this data set."<<endl;

             continue;

         }

         cout<<setiosflags(ios::fixed)<<setprecision()<<sqrt(((sum_all*6.0)/sum))<<endl;

     }

 }

 int gcd(int a,int b)

 {

     if(b == )

         return a;

     return gcd(b,a%b);

 }