#include <bits/stdc++.h>
#define REP(i,a,b) for(int i=(a);i<(b);i++)
#define RREP(i,a,b) for(int i=(a);i>=(b);i--)
#define pq priority_queue
#define P pair<int,int>
#define P2 pair<int,P>
#define P3 pair<int,P2>
typedef long long ll; typedef long double ld;
using namespace std;
const int INF=1e9, MOD=1e9+7, around[]={0,1,1,-1,-1,0,-1,1,0,0};
const ll LINF=1e18;
const ld PI=abs(acos(-1));
int n,a[200010];
ll k;

bool c(double t){
	ll pos=0;
	REP(i,0,n) pos+=a[i]/t;
	
	return (pos>=k?1:0);
}

double binary(){
	double ok=0.00000000001, ng=(double)INF;
	
	while(abs(ok-ng)>0.00000000001){
		double mid=(ok+ng)/2;
		if(c(mid)) ok=mid;
		else ng=mid;
	}
	
	return ok;
}

int main(){
	cin >> n;
	REP(i,0,n) cin >> a[i];
	cin >> k;
	
	cout << fixed << setprecision(10);
	cout << binary() << endl;
	return 0;
}