import copy
from functools import lru_cache
import sys
sys.setrecursionlimit(1000000)

N = int(input())
A = list(map(int,input().split()))

B = [min(A[i],3) for i in range(N)]
B = tuple(B)
#print(B)

#0,1,2,3以上個の数
C = [0,0,0,0]
for x in A:
    if x >=3 :
        C[3] += 1
    else:
        C[x] += 1
C = tuple(C)
#print(C)

@lru_cache(maxsize=None)
def solve(X):
    ret = N
    loop = N - X[3]
    #print(X)
    for i in range(3):
        if X[i] == 0: continue
        if i <= 2:
            Z = list(X)
            Z[i] -= 1
            Z[i+1] += 1
            Z = tuple(Z)
            ret += solve(Z)*X[i]
    #print("A",X,loop,ret)
    if loop == 0:
        ret = 0
    else:
        ret /= loop
    #print(X,loop,ret)
    return ret

ans = solve(C)
print(ans)