#include <bits/stdc++.h>

typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define DECIM8  fixed<<setprecision(8) 
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

using namespace std;
int main(void)
{
  int n;
  cin >> n;
  vector<double> l;
  l.resize(n);
  double left=0,right=0;
  REP(i,n) cin >> l[i];
  LL k;
  cin >> k;
  REP(i,n) left=max(left,l[i]);
  left/=k;
  REP(i,n) right+=l[i];
  right/=k;
  while(right-left>EPS&&right>left*(1+EPS)){
    double tmp=(left+right)/2;
    LL stick=0;
    REP(i,l.size()) stick+=l[i]/tmp;
    if(stick<k) right=tmp;
    else left=tmp;
  }
  cout << fixed<<setprecision(10) << (left+right)/2 << endl;
  return 0;
}