N = int(input())
A = list(map(int,input().split()))

import sys
sys.setrecursionlimit(10 ** 8)
from functools import lru_cache
@lru_cache(maxsize = 1000)
def calc(a,b,c):
    if a == 0 and b == 0 and c == 0:return 0
    u =a + b + c
    tmp = 0
    if a != 0:
        tmp += calc(a-1,b,c) * a
    if b != 0:
        tmp += calc(a+1,b-1,c) * b
    if c != 0:
        tmp += calc(a,b+1,c-1) * c
    tmp = (tmp + N) / u
    return tmp
a = 0
b = 0
c = 0
for i in A:
    if i == 0:c += 1
    elif i == 1:b += 1
    elif i == 2:a += 1
print(calc(a,b,c))