import sys
sys.setrecursionlimit(10**9)

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

dp = [[[-1]*(110) for _ in range(110)] for _ in range(110)]

def rec(i,j,k):
    if dp[i][j][k] != -1:
        return dp[i][j][k]
    if i+j+k == 0:
        return 0.0
    
    ret = 0
    if i > 0:
        ret += i*rec(i-1,j+1,k)
    if j > 0:
        ret += j*rec(i,j-1,k+1)
    if k > 0:
        ret += k*rec(i,j,k-1)
    ret += n
    ret *= 1.0/(i+j+k)
    
    dp[i][j][k] = ret
    return dp[i][j][k]

I = 0
J = 0
K = 0
for a in A:
    if a == 0:
        I += 1
    elif a == 1:
        J += 1
    elif a == 2:
        K += 1
        
print(rec(I,J,K))