from functools import lru_cache

@lru_cache(maxsize=None)
def solve(k0, k1, k2, k3):
    if any([k < 0 for k in [k0, k1, k2]]):
        return -1
    
    n = sum([k0, k1, k2, k3])
    if (k0, k1, k2, k3) == (0, 0, 0, n):
        return 0
    
    s = k3 / n
    s += (solve(k0 - 1, k1 + 1, k2, k3) + 1) * (k0 / n)
    s += (solve(k0, k1 - 1, k2 + 1, k3) + 1) * (k1 / n)
    s += (solve(k0, k1, k2 - 1, k3 + 1) + 1) * (k2 / n)
    s *= n / (n - k3)
    
    return s

n = int(input())
a_ls = [int(i) for i in input().split()]

k0 = a_ls.count(0)
k1 = a_ls.count(1)
k2 = a_ls.count(2)
k3 = n - k0 - k1 - k2

e = solve(k0, k1, k2, k3)
print(e)