UVa 412 - Pi

///     Raihan Ruhin
///     CSE, Jahangirnagar University.
///     Dhaka-Bangladesh.
///     id: raihanruhin (topcoder / codeforces / codechef / uva), 3235 (lightoj)
///     mail: raihanruhin@ (yahoo / gmail / facebook)
///     blog: ruhinraihan.blogspot.com

#include<bits/stdc++.h>
using namespace std;

#define SET(a) memset(a,-1,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))
#define PI acos(-1.0)

#define MOD 1000000007
#define MX 100010

int main()
{
    ios_base::sync_with_stdio(0);cin.tie(0);
    int tc,kk=1, n, arr[55];
    while(cin>>n && n)
    {
        for(int i=0;i<n;i++)    cin>>arr[i];
        int cnt=0, tot=n*(n-1)/2*6;
        for(int i=0;i<n-1;i++)
            for(int j=i+1;j<n;j++)
                if(__gcd(arr[i], arr[j])==1)
                    cnt++;
        if(!cnt) cout<<"No estimate for this data set.\n";
        else cout<<setprecision(6)<<fixed<<sqrt((double)tot/(double)cnt)<<"\n";
    }


    return 0;
}