#include<cstdio>
#include<vector>


using namespace std;
using ll=long long int;


int main(void)
{
	int i, j, n;
	ll k, sum;
	vector<long double> l;
	long double l_max, s, e, c, ans;

	while(scanf("%d", &n)==1)
	{
		l_max=0.0;
		l.resize(n);
		for(i=0;i<n;i++)
		{
			ll lt;
			scanf("%lld", &lt);
			l[i]=lt+1e-10;
			if(l_max<l[i]) l_max=l[i];
		}
		scanf("%lld", &k);

		s=l_max/k;
		e=l_max;
		ans=s;
		for(j=0;j<50;j++)
		{
			c=(s+e)*0.5;
			sum=0;
			for(i=0;i<n;i++)
			{
				sum+=l[i]/c;
			}
			if(sum>=k)
			{
				ans=c;
				s=c;
			}
			else
			{
				e=c;
			}
		}
		printf("%.16Lf\n", ans);
	}

	return 0;
}