from collections import defaultdict


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

    n3 = n2 = n1 = 0
    for a in A:
        if a == 0:
            n3 += 1
        elif a == 1:
            n2 += 1
        elif a == 2:
            n1 += 1

    memo = defaultdict(int)

    def rec(i, j, k):
        if i < 0 or j < 0 or k < 0:
            return 0

        ijk = (i + j + k)
        if ijk == 0:
            return 0

        if (i, j, k) in memo:
            return memo[i, j, k]

        ans = (
            N / ijk +
            rec(i, j, k - 1) * k / ijk +
            rec(i, j - 1, k + 1) * j / ijk +
            rec(i - 1, j + 1, k) * i / ijk
        )
        memo[i, j, k] = ans
        return ans

    print(rec(n3, n2, n1))


def main():
    solve()


if __name__ == '__main__':
    main()