# -*- coding: utf-8 -*-

dp = [[[-1] * 101 for j in range(101)] for i in range(101)]

def solve(a, b, c):
    s = a + b + c
    if s == 0:
        dp[a][b][c] = 0
    if dp[a][b][c] >= 0:
        return dp[a][b][c]

    # a, b, cのいずれかが出る(自己ループを抜ける)までの回数の期待値
    dp[a][b][c] = N / s
    if a > 0:
        dp[a][b][c] += solve(a-1, b+1, c) * a / s
    if b > 0:
        dp[a][b][c] += solve(a, b-1, c+1) * b / s
    if c > 0:
        dp[a][b][c] += solve(a, b, c-1) * c / s

    return dp[a][b][c]


N = int(input())
A = list(map(int, input().split()))
num = [0, 0, 0]
for x in A:
    if x < 3:
        num[x] += 1
print(solve(num[0], num[1], num[2]))