from itertools import combinations

n = int(input())
values = list(map(int, input().split()))

ans = 0


def dfs(v):
    if len(v) == 2:
        global ans
        ans = max((v[0] + v[1]) / 2, ans)
        return

    for vv in combinations(v, 2):
        v_alche = v[:]
        v_alche.remove(vv[0])
        v_alche.remove(vv[1])
        v_alche.append((vv[0] + vv[1]) / 2)
        dfs(v_alche)


dfs(values)
print(ans)
