from typing import List, Tuple, Callable, TypeVar
import sys
import itertools
import heapq
import bisect
import math
from collections import deque, defaultdict, Counter
from functools import lru_cache, cmp_to_key

input = sys.stdin.readline

if __file__ != 'prog.py':
    sys.setrecursionlimit(10 ** 6)


def readints(): return map(int, input().split())
def readlist(): return list(readints())
def readstr(): return input()[:-1]


N = int(input())
A = readlist()
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1


@lru_cache(None)
def solve(a, b, c):
    if a + b + c == 0:
        return 0
    acc = 0
    if a:
        acc += a * solve(a - 1, b + 1, c)
    if b:
        acc += b * solve(a, b - 1, c + 1)
    if c:
        acc += c * solve(a, b, c - 1)
    return (acc + N) / (a + b + c)


print(solve(cnt[0], cnt[1], cnt[2]))