import sys
sys.setrecursionlimit(200050)

def code(X):
    ret = 0
    for i in range(4):
        ret *= (N + 1)
        ret += X[i]
    return ret

def decode(x: int):
    RET = [0] * 4
    for i in reversed(range(4)):
        RET[i] = x % (N + 1)
        x //= (N + 1)
    return RET

N = int(input())
A = list(map(int, input().split()))
ST = [0] * 4
for a in A:
    ST[min(a, 3)] += 1

from functools import lru_cache
@lru_cache(maxsize=20000)
def dfs(x: int):
    if x == N:
        return 0
    a, b, c, d = map(int, decode(x))
    ret = 1 / (1 - d / N)
    if a > 0:
        ret += dfs(code([a - 1, b + 1, c, d])) * a / (N - d)
    if b > 0:
        ret += dfs(code([a, b - 1, c + 1, d])) * b / (N - d)
    if c > 0:
        ret += dfs(code([a, b, c - 1, d + 1])) * c / (N - d)
    print((a, b, c, d), ret)
    return ret


print(dfs(code(ST)))