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#include <iostream>
#include <cstdio>
#include <cmath>
#define LEN 52
using namespace std;
unsigned int gcd( unsigned int , unsigned int );

/*
這論壇不錯阿 大家都不用@@
http://online-judge.uva.es/board/viewtopic.php?f=5&t=8221&p=102632&hilit=412&sid=a479f43c6e4962712ced715f3c475cd9#p102632
我的UVa進不去 網路太慢 所以用ZJ測了
http://zerojudge.tw/RealtimeStatus?account=x000032001&status=AC
有問題再看看吧?
*/
int main()
{
    int n; //設定要輸入幾組數，最多50

    while(  cin >> n && n )
    {
        unsigned int s[LEN] = {};

        for( int i = 0 ; i < n ; i++)
            cin >> s[i];  //將所輸入的數丟進 s[i] 中

        unsigned int k = 0; //算所輸入的數中，互質的個數
        for ( int i = 0 ; i < n ; ++i )
            for ( int j = i + 1 ; j < n ; ++j )
            {
                if ( gcd( s[ i ], s[ j ] ) == 1 )  //互質的話，k+1
                    k++;
            }
        if ( k == 0 )
            cout << "No estimate for this data set." << endl;
        else
        {

            double pi = sqrt( (n-1)*(double)n*3/(double)k );
            printf("%.6f\n",pi);
        }
    }
    return 0;
}


unsigned int gcd( unsigned int a , unsigned int b )//求最小公因數
{
    while(1)
    {
        a = a%b;
            if(a==0)return b;
        b = b%a;
            if(b==0)return a;

    }
}