N=int(raw_input())
A=map(int,raw_input().split())
needs=[0 for i in range(4)]
dp=[[[-1.0 for i in range(100)]for j in range(100)]for k in range(100)]

def rec(a,b,c):
    global dp
    if a==0 and b==0 and c==0:
        return 0
    if a<0 or b<0 or c<0:
        return 0
    if dp[a][b][c]!=-1.0:
        return dp[a][b][c]
    dp[a][b][c]=0.0
    dp[a][b][c]+= ( N*1.0/(a+b+c)+rec(a-1,b,c))* a*1.0/(a+b+c) 
    dp[a][b][c]+= ( N*1.0/(a+b+c)+rec(a+1,b-1,c))* b*1.0/(a+b+c) 
    dp[a][b][c]+= ( N*1.0/(a+b+c)+rec(a,b+1,c-1))* c*1.0/(a+b+c) 
    #print a,b,c,dp[a][b][c]
    return dp[a][b][c]

for a in A:
    if a<3:
        needs[3-a]+=1
print rec(needs[1],needs[2],needs[3])