import sys

sys.setrecursionlimit(10 ** 6)
from bisect import *
from collections import *
from heapq import *

int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def MI1(): return map(int1, sys.stdin.readline().split())
def MF(): return map(float, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LF(): return list(map(float, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
dij = [(0, 1), (1, 0), (0, -1), (-1, 0)]

def main():
    n = II()
    aa = LI()
    c0 = c1 = c2 = 0
    for a in aa:
        if a == 0: c0 += 1
        if a == 1: c1 += 1
        if a == 2: c2 += 1
    # dp[i][j][k]…0枚なのがi種類、1枚なのがj種類、2枚なのがk種類のとき、あと何回引くかの期待値
    dp = [[[0] * (c0 + c1 + c2 + 1) for _ in range(c0 + c1 + 1)] for _ in range(c0 + 1)]
    for i in range(c0 + 1):
        for j in range(c0 + c1 + 1):
            for k in range(c0 + c1 + c2 + 1):
                if (i, j, k) == (0, 0, 0): continue
                v = n
                if i - 1 >= 0 and j + 1 <= c0 + c1: v += dp[i - 1][j + 1][k] * i
                if j - 1 >= 0 and k + 1 <= c0 + c1 + c2: v += dp[i][j - 1][k + 1] * j
                if k - 1 >= 0: v += dp[i][j][k - 1] * k
                dp[i][j][k] = v / (i + j + k)
    print(dp[c0][c1][c2])

main()

# dp[7][5][3] dp[6][6][3] dp[7][4][4] dp[7][5][2]