def aa(a,b,c):
    if a+b+c==0:
        return 0.0
    if (a,b,c) in v:
        return v[(a,b,c)]
    global n
    res=n/(a+b+c)
    m=a+b+c
    if a:
        res+=aa(a-1,b+1,c)/m*a
    if b:
        res+=aa(a,b-1,c+1)/m*b
    if c:
        res+=aa(a,b,c-1)/m*c
    v[(a,b,c)]=res
    return res

n=int(input())
v={}
a,b,c=0,0,0
for i in list(map(int,input().split())):
    if i==0:
        a+=1
    if i==1:
        b+=1
    if i==2:
        c+=1
print(aa(a,b,c))