n = int(input())
*a, = map(int,input().split())

dp = [[[0]*(n+1) for _ in range(n+1)] for _ in range(n+1)]
for i in range(n+1):
    for j in range(n+1):
        for k in range(n+1):
            if i==j==k==0: continue
            x = n-i-j-k
            if x < 0: break
            v = 0.0
            if i: v += i*dp[i-1][j+1][k]
            if j: v += j*dp[i][j-1][k+1]
            if k: v += k*dp[i][j][k-1]
            dp[i][j][k] = (v/n + 1)*n/(n-x)

r = [0,0,0]
for ai in a:
    if ai <= 2: r[ai] += 1
print(dp[r[0]][r[1]][r[2]])