import std.stdio;
import std.array;
import std.string;
import std.conv;
import std.algorithm;

void main() {
  int N = to!int(chomp(readln()));
  int[] L = array(map!(to!int)(split(readln())));
  int K = to!int(chomp(readln()));

  real high = 1e9;
  real low = 0.0;
  real middle;
  int s;
  for (int i = 0; i < 80; i++) {
    s = 0;
    middle = (high+low)/2.0;
    for (int j = 0; j < N; j++)
      s += to!int(to!real(L[j])/middle);
    if (s >= K)
      low = middle;
    else
      high = middle;
  }
  writefln("%.9f", high);
}