n = int(input())
arr = list(map(int,input().split()))

cnt = [0] * 4
for i in range(n):
    if arr[i] >= 3:
        cnt[3] += 1
    else:
        cnt[arr[i]] += 1

dp = [[[0 for i in range(n + 1)] for j in range(n + 1)] for k in range(n + 1)]

dp[0][0][0] = 0

for a in range(n + 1):
    for b in range(n + 1):
        for c in range(n + 1):
            if a + b + c == 0:
                continue

            dp[a][b][c] = n / (a + b + c)

            if a - 1 >= 0 and b + 1 <= n:
                dp[a][b][c] += a / (a + b + c) * dp[a - 1][b + 1][c]
            if b - 1 >= 0 and c + 1 <= n:
                dp[a][b][c] += b / (a + b + c) * dp[a][b - 1][c + 1]
            if c - 1 >= 0:
                dp[a][b][c] += c / (a + b + c) * dp[a][b][c - 1]

print(dp[cnt[0]][cnt[1]][cnt[2]])