N = int(input())
A = list(map(int, input().split()))
dp = [[[-1] * (N + 1) for _ in range(N + 1)] for _ in range(N + 1)]
dp[N][N][N] = 0
for k in range(N, -1, -1):
    for j in range(N, k - 1, -1):
        for i in range(N, j - 1, -1):
            if k == N:
                continue
            elif j == N:
                dp[i][j][k] = (1 + dp[i][j][k + 1] * (j - k) / N) * N / (N - k)
            elif j == N:
                dp[i][j][k] = (
                    (dp[i + 1][j][k] * (N - i) / N + dp[i][j][k + 1] * (j - k) / N)
                    * N
                    / (N - k)
                )
            elif i == N:
                dp[i][j][k] = (
                    (1 + dp[i][j + 1][k] * (i - j) / N + dp[i][j][k + 1] * (j - k) / N)
                    * N
                    / (N - k)
                )
            else:
                dp[i][j][k] = (
                    (
                        1
                        + dp[i + 1][j][k] * (N - i) / N
                        + dp[i][j + 1][k] * (i - j) / N
                        + dp[i][j][k + 1] * (j - k) / N
                    )
                    * N
                    / (N - k)
                )


i, j, k = 0, 0, 0
for a in A:
    if a >= 3:
        i += 1
        j += 1
        k += 1
    elif a >= 2:
        i += 1
        j += 1
    elif a >= 1:
        i += 1
print(dp[i][j][k])