N=int(input())
A=list(map(int,input().split()))
X=[0,0,0,0]

for a in A:
    X[min(3,a)]+=1

DP=[[[0]*(N+1) for i in range(N+1)] for j in range(N+1)]
P=[[[0]*(N+1) for i in range(N+1)] for j in range(N+1)]

DP[X[0]][X[1]][X[2]]=0
P[X[0]][X[1]][X[2]]=1

for rest in range(N,-1,-1):
    for i in range(N,-1,-1):
        for j in range(N,-1,-1):
            k=rest-i-j
            if k<0:
                continue
            
            if P[i][j][k]<=0:
                continue

            if i>0:
                if P[i-1][j+1][k]==0:
                    P[i-1][j+1][k]=P[i][j][k]*i/(i+j+k)
                    DP[i-1][j+1][k]=DP[i][j][k]+N/(i+j+k)
                else:
                    xx=P[i-1][j+1][k]
                    yy=P[i][j][k]*i/(i+j+k)

                    DP[i-1][j+1][k]=(xx*DP[i-1][j+1][k]+yy*(DP[i][j][k]+N/(i+j+k)))/(xx+yy)
                    P[i-1][j+1][k]+=yy

            if j>0:                
                if P[i][j-1][k+1]==0:
                    P[i][j-1][k+1]=P[i][j][k]*j/(i+j+k)
                    DP[i][j-1][k+1]=DP[i][j][k]+N/(i+j+k)
                else:
                    xx=P[i][j-1][k+1]
                    yy=P[i][j][k]*j/(i+j+k)

                    DP[i][j-1][k+1]=(xx*DP[i][j-1][k+1]+yy*(DP[i][j][k]+N/(i+j+k)))/(xx+yy)
                    P[i][j-1][k+1]+=yy

            if k>0:
                if P[i][j][k-1]==0:
                    P[i][j][k-1]=P[i][j][k]*k/(i+j+k)
                    DP[i][j][k-1]=DP[i][j][k]+N/(i+j+k)
                else:
                    xx=P[i][j][k-1]
                    yy=P[i][j][k]*k/(i+j+k)

                    DP[i][j][k-1]=(xx*DP[i][j][k-1]+yy*(DP[i][j][k]+N/(i+j+k)))/(xx+yy)
                    P[i][j][k-1]+=yy

print(DP[0][0][0])