import sys
readline=sys.stdin.readline

N=int(readline())
dp=[[[0]*(N+1) for j in range(N+1)] for i in range(N+1)]
for  ijk in range(1,N+1):
    for i in range(ijk+1):
        jk=ijk-i
        for j in range(jk+1):
            k=jk-j
            dp[i][j][k]=N/(i+j+k)
            if i:
                dp[i][j][k]+=dp[i-1][j+1][k]*i/(i+j+k)
            if j:
                dp[i][j][k]+=dp[i][j-1][k+1]*j/(i+j+k)
            if k:
                dp[i][j][k]+=dp[i][j][k-1]*k/(i+j+k)
A=list(map(int,readline().split()))
c0,c1,c2=0,0,0
for a in A:
    if a==0:
        c0+=1
    elif a==1:
        c1+=1
    elif a==2:
        c2+=1
ans=dp[c0][c1][c2]
print(ans)