N = int(input())
A = list(map(int, input().split()))
X = [0] * 4
for a in A:
    X[min(a, 3)] += 1
    
dp = [[[0] * (N + 5) for i in range(N + 5)] for _ in range(N + 5)]
for n in range(1, N + 1):
    for z in range(n, -1, -1):
        for y in range(n - z, -1, -1):
            x = n - y - z
            s = x + y + z
            dp[x][y][z] = dp[x-1][y+1][z] * x + dp[x][y-1][z+1] * y + dp[x][y][z-1] * z + N
            dp[x][y][z] /= s
            
print(dp[X[0]][X[1]][X[2]])