from collections import Counter
N = int(input())
A = list(map(int, input().split()))
need = [max(3-a, 0) for a in A]
cnt = Counter(need)

# コンプまであと(1,2,3)枚必要なカード数がdp(a, b, c)
# ここからdp(0,0,0)にするまでカードを買う回数の期待値
U = 3 * N + 1
dp = [[[0.0] * U for _ in range(U)] for _ in range(U)]

for c in range(U):
    for b in range(U):
        if c + b > U:
            break
        for a in range(U):
            if c + b + a > U:
                break
            if a + b + c == 0:
                continue
            E = N
            p = 0
            if a > 0:
                E += dp[a - 1][b][c] * a
                p += a
            if b > 0 and a < N:
                E += dp[a + 1][b - 1][c] * b
                p += b
            if c > 0 and b < N:
                E += dp[a][b + 1][c - 1] * c
                p += c
            dp[a][b][c] = E / p

x = cnt[1]
y = cnt[2]
z = cnt[3]
print(dp[x][y][z])