EPS = 1.0e-11

n = int(input())
l = list(map(int, input().split()))
k = int(input())

ok = EPS
ng = 1.0e9 + EPS
while abs(ok - ng) > EPS:
  mid = (ok + ng) / 2

  cnt = 0
  for i in range(n):
    ok2 = 0
    ng2 = k + 1
    while abs(ok2 - ng2) > 1:
      mid2 = (ok2 + ng2) // 2

      if mid2 * mid <= l[i]:
        ok2 = mid2
      else:
        ng2 = mid2

    cnt += ok2

  if cnt >= k:
    ok = mid
  else:
    ng = mid

print(f"{ok:.12f}")