#include <iostream>
#include <cstdio>

int N, K;
double L[200000];

bool can(double x){
    int k = 0;
    for(int i=0;i<N;i++){
        k += L[i] / x;
    }

    return k >= K;
}

int main(){
    std::cin >> N;

    for(int i=0;i<N;i++){
        std::cin >> L[i];
    }

    std::cin >> K;

    double lb = 0.0f, ub = 1000000100.0f;
    for(int i=0;i<500;i++){
        double mid = (lb+ub) / 2;
        if(can(mid)){lb = mid;}
        else{ub = mid;}
    }

    printf("%.10f\n", lb);
}