import bisect
import os
from collections import Counter, deque
from fractions import gcd
from functools import lru_cache
from functools import reduce
import functools
import heapq
import itertools
import math
import numpy as np
import re
import sys

if os.getenv("LOCAL"):
    sys.stdin = open("_in.txt", "r")

sys.setrecursionlimit(2147483647)
INF = float("inf")

N = int(sys.stdin.readline())
A = np.array(list(map(int, sys.stdin.readline().split())))
reqs = np.maximum(0, 3 - A)


@lru_cache(maxsize=None)
def expected(i, j, k):
    # あと 1 個必要なカードが i 種類
    # あと 2 個必要なカードが j 種類
    # あと 3 個必要なカードが k 種類
    if i == j == k == 0:
        return 0
    s = 0
    if i > 0:
        s += expected(i - 1, j, k) * i
    if j > 0:
        s += expected(i + 1, j - 1, k) * j
    if k > 0:
        s += expected(i, j + 1, k - 1) * k
    return s / (i + j + k) + N / (i + j + k)


print(expected((reqs == 1).sum(), (reqs == 2).sum(), (reqs == 3).sum()))