#include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
using namespace std;

#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP(i,n,0)
#define REP(i,s,e) for (i = s; i < e; i++)
#define rep(i,n) REP(i,0,n)
#define INF 100000000

typedef long long ll;

int main() {
    int i, n;
    long double l, r;
    ll k;
    int L[200000];
    cin >> n;
    rep (i,n) cin >> L[i];
    cin >> k;
    l = 1e-10; r = 1000000000;
    while (l < r) {
        long double m = (l+r) / 2;
        ll sum = 0;
        rep (i,n) sum += L[i]/m;
        if (sum < k)
            r = m-1e-10;
        else
            l = m+1e-10;
    }
    cout << fixed << setprecision(10) << r << endl;
    return 0;
}