n=int(input())
d=list(map(int,input().split()))
dp=[[[0.0]*(n+1) for i in range(n+1)] for i in range(n+1)]
for i in range(n+1):
    for j in range(n+1):
        if i+j>n:
            break
        for l in range(n+1):
            if i+j+l>n:
                break
            if i+j+l==n:
                dp[i][j][l]=1.0
            elif i+j+l>0:
                dp[i][j][l]=n/(n-i-j-l)
            m=i+j+l
            if i:
                dp[i][j][l]+=dp[i-1][j+1][l]/m*i
            if j:
                dp[i][j][l]+=dp[i][j-1][l+1]/m*j
            if l:
                dp[i][j][l]+=dp[i][j][l-1]/m*l
a,b,c=0,0,0
for i in d:
    if i==1:
        b+=1
    if i==2:
        c+=1
    if i==0:
        a+=1
print(dp[a][b][c])