n = int(input())
v = list(map(int, input().split()))
ans = 0
for i in range(n-1):
    for j in range(i+1, n):
        p = (v[i] + v[j]) / 2
        for k in range(n):
            if k == i or k == j: continue
            p = (p + v[k]) / 2
        ans = max(ans, p)
print(ans)
