import itertools

N = int(input())
li = list(map(int, input().split()))
t = []

max = 0

for i in itertools.permutations(range(N)):
  t.append(i)

for i in t:
  count = 0
  for n in range(N-1):
    if n == 0:
      count = (li[i[n]] + li[i[n+1]]) / 2
    else:
      count = (count + li[i[n+1]]) / 2

  if max < count:
    max = count

print(max)