from functools import lru_cache
import sys

sys.setrecursionlimit(4100000)
n = int(input())
A = list(map(int, input().split()))

n1, n2, n3 = 0, 0, 0
for a in A:
    a = min(a, 3)
    if a == 0:
        n3 += 1
    if a == 1:
        n2 += 1
    if a == 2:
        n1 += 1


@lru_cache(None)
def calc(a, b, c):
    if a == 0 and b == 0 and c == 0:
        return 0

    res = n
    goukei = a + b + c
    if c:
        res += (calc(a, b + 1, c - 1)) * c
    if b:
        res += (calc(a + 1, b - 1, c)) * b
    if a:
        res += (calc(a - 1, b, c)) * a
    res /= goukei
    return res


print(calc(n1, n2, n3))