#include <iostream>
#include <iomanip>
#include<math.h>
using namespace std;
long long int N,i=0;
long double K;
long double L[300000];
void nibutan(long double s,long double t);
bool f(long double x);

int main(){
    //long double s,t;
    cin>>N;
    for(int j=0;j<N;j++){
        cin>>L[j];
    }
    cin>>K;
    nibutan(0.000000001,1000000000);
}

void nibutan(long double s,long double t){
long double ng = s, ok = t;
while (i<100) {
    long double mid = (ng + ok) / 2;
     (f(mid) ? ok : ng) = mid;
    //cout<<mid<<endl;
     i++;
}
    fixed;
    cout<<setprecision(10)<<ok<<endl;
}

bool f(long double x){
    long long int s=0;
    for(int j=0;j<N;j++){
        s+=L[j]/x;
    }
    if(K>s)return true;
    else return false;
}