#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines


# %%
N, *A = map(int, read().split())

# %%
a = sum(x == 0 for x in A)
b = sum(x == 1 for x in A)
c = sum(x == 2 for x in A)


# %%
dp = [[[0] * (N + 2) for _ in range(N + 2)] for _ in range(N + 2)]
for n in range(N + 1):
    for i in range(n + 1):
        for j in range(n + 1):
            if i == j == n == 0:
                continue
            k = n - i - j
            if k < 0:
                break
            x = N + i * dp[i - 1][j + 1][k] + j * dp[i][j - 1][k + 1] + k * dp[i][j][k - 1]
            dp[i][j][k] = x / (i + j + k)


# %%
print(dp[a][b][c])