from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline


N = int(input())
A = list(map(int,input().split()))
X = [0]*4
for a in A:
    m = min(a,3)
    X[m] += 1
    
mem = defaultdict(lambda:-1)
mem[(0,0,0,N)]=0
def f(X):
    if mem[X]!=-1:
        return mem[X]
    Z = list(X)
    
    tmp = 0
    for i in range(3):
        
        if Z[i]==0:
            continue
        
        Z[i+1] += 1
        Z[i] -= 1
        tmp += (Z[i]+1)*(1 + f(tuple(Z)))
        Z[i+1] -= 1
        Z[i] += 1
    tmp += Z[3]
    tmp /= (N-Z[3])
    mem[tuple(X)] = tmp
    return tmp
    
print(f(tuple(X)))