#include<bits/stdc++.h>

using namespace std;

/// Typedef
typedef long long ll;

#define sc1(a) scanf("%lld",&a)
#define sc2(a,b) scanf("%lld %lld",&a,&b)

#define pf1(a) printf("%lld\n", a)
#define pf2(a,b) printf("%lld %lld\n",a,b)

#define mx 10000007
#define mod 1000000007
#define PI acos(-1.0)

#define size1 1000007

int drx[8] = {-2,-2,-1,-1,1,1,2,2};
int dcy[8] = {-1,1,-2,2,-2,2,-1,1};

int dirx[4] = { -1, 0, 1, 0 };
int diry[4] = { 0, -1, 0, 1 };


ll gcd(ll a,ll b){ if(b == 0) return a; return gcd(b, a % b); }
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}


int main()
{

    ll tc, num, t = 1, pownum;

    // freopen("/opt/Coding/clion code/input.txt", "r", stdin);
    // freopen("/opt/Coding/clion code/output.txt", "w", stdout);

    while (cin >> num){

        if(num <= 1 || num >= 50) break;

        ll arr[num];

        for(ll i = 0; i < num; i++)
            sc1(arr[i]);

        ll cnt = 0;
        for(ll i = 0; i < num - 1; i++){
            for(ll j = i + 1; j < num; j++){
                if(gcd(arr[i], arr[j]) == 1) cnt++;
            }
        }

        if(cnt == 0) cout << "No estimate for this data set." << endl;
        else{

            ll sum = num * (num - 1) / 2;

            double ans = sqrt((6.0 * sum) / cnt);

            printf("%.6lf\n", ans);

        }

    }

    return 0;
}