from collections import Counter
N = int(input())
counter = Counter(3-n for n in map(int, input().split()) if n < 3)

dp = [[[0]*(N+2) for _ in [0]*(N+2)] for _ in [0]*(N+2)]


def solve(rem1, rem2, rem3):
    necessary = rem1 + rem2 + rem3
    if necessary == 0 or necessary > N:
        return 0

    expectation = N / necessary

    if not dp[rem1][rem2][rem3]:
        dp[rem1][rem2][rem3] = (
            (solve(rem1-1, rem2, rem3) * rem1 / necessary if rem1 else 0)
            + (solve(rem1+1, rem2-1, rem3) * rem2 / necessary if rem2 else 0)
            + (solve(rem1, rem2+1, rem3-1) * rem3 / necessary if rem3 else 0)
            + expectation
        )

    return dp[rem1][rem2][rem3]


print(solve(counter[1], counter[2], counter[3]))