#include<bits/stdc++.h>
using namespace std;

typedef long long ll;


int N;
int L[200000];
ll K;

ll f(double x){
    double d=1.0/x;
    ll ret=0;
    for(int i=0;i<N;i++)ret+=(ll)(L[i]*d);
    return ret;
}
int main(){
    scanf("%d",&N);
    for(int i=0;i<N;i++)scanf("%d",&L[i]);
    scanf("%lld",&K);

    double lb=0,ub=1e10;
    for(int i=0;i<100;i++){
        double mid=(ub+lb)/2;
        if(f(mid)>=K)lb=mid;
        else ub=mid;
    }
    printf("%.10lf\n",(ub+lb)/2);

    return 0;

}