import sys
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline

N,*L,K=map(int,read().split())

l,r=0,10**9
for _ in range(100):
  m=(l+r)/2
  cnt=sum(l//m for l in L)
  if cnt>=K:l=m
  else:r=m
print("{:.15f}".format(l))