#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <string>
#include <iomanip>
#include <map>
#include <set>
#include <cmath>
#include <cstdio>
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define FOR(i,m,n) for(int (i)=(m);(i)<(n);(i)++)
#define All(v) (v).begin(),(v).end()
typedef long long ll;

ll calc(double v,vector<ll> &l){
    ll ret = 0;
    rep(i,l.size()){
        ret+=l[i]/v;
    }
    return ret;
}

int main(){
    ll N;cin >> N;
    vector<ll> L(N);
    rep(i,N)cin >> L[i];
    ll K;cin >> K;
    double l=0,r=1e11;
    double m;
    rep(i,100){
        m=(l+r)/2;
        if(calc(m,L)>=K)l=m;
        else r=m;
    }
    cout << fixed << setprecision(12) << m << endl;
    return 0;
}