import sys
from functools import lru_cache
sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N = I()
A = LI()
x,y,z = 0,0,0
for a in A:
    if a == 0:
        x += 1
    elif a == 1:
        y += 1
    elif a == 2:
        z += 1


@lru_cache(maxsize=None)
def dp(i,j,k):  # 0,1,2枚集まっているカードがi,j,k種類ある状態での期待値
    if i < 0 or j < 0 or k < 0:
        return 0
    if i == j == k == 0:
        return 0
    return (i*dp(i-1,j+1,k)+j*dp(i,j-1,k+1)+k*dp(i,j,k-1)+N)/(i+j+k)


print(dp(x,y,z))