def list3(a, b, c, *, val=0):
    return [[[val] * c for _ in range(b)] for _ in range(a)]


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


# 0枚がa種。1枚がb種、2枚がc種
def f(a, b, c):
    if a+b+c == 0: return 0
    if memo[a][b][c] != 0: return memo[a][b][c]

    res = 1
    if a > 0: res += f(a-1, b+1, c) * a / N
    if b > 0: res += f(a, b-1, c+1) * b / N
    if c > 0: res += f(a, b, c-1) * c / N
    res *= N / (a+b+c)
    memo[a][b][c] = res
    return res


a = A.count(0)
b = A.count(1)
c = A.count(2)
memo = list3(N+1, N+1, N+1, val=0)
ans = f(a, b, c)
print(ans)