import sys


def input(): return sys.stdin.readline().rstrip()


def main():
    N = int(input())
    *A, = map(int, input().split())
    c0, c1, c2 = A.count(0), A.count(1), A.count(2)

    DP = [[[None]*(c0+c1+c2-(i+j)+1) for j in range(c1+c0-i+1)]
          for i in range(c0+1)]

    def solve(a, b, c):
        if a == b == c == 0:
            return 0

        if DP[a][b][c] is not None:
            return DP[a][b][c]

        ans = 0
        if a > 0:
            ans += a * solve(a-1, b+1, c)
        if b > 0:
            ans += b * solve(a, b-1, c+1)
        if c > 0:
            ans += c * solve(a, b, c-1)

        DP[a][b][c] = (ans + N) / (a+b+c)
        return DP[a][b][c]

    print(solve(c0, c1, c2))


if __name__ == '__main__':
    main()