n = int(input())

a = [0, 0, 0]
for v in map(int, input().split()):
    if v <= 2:
        a[v] += 1

dp = [[[0 for _ in range(n+3)] for _ in range(n+3)] for _ in range(n+3)]

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

            dp[i][j][k] += n/nonzero
            if (i-1 >= 0): dp[i][j][k] += dp[i-1][j+1][k] * i/nonzero
            if (j-1 >= 0): dp[i][j][k] += dp[i][j-1][k+1] * j/nonzero
            if (k-1 >= 0): dp[i][j][k] += dp[i][j][k-1] * k/nonzero

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