def main():
  N = int(input())
  A = [int(val) for val in input().split()]
  dp = [[[0 for i in range(N + 5)] for j in range(N + 5)] for k in range(N + 5)]

  freqs = [0] * 4
  for a in A:
    freqs[min(3, a)] += 1

  a, b, c = freqs[1:]

  for i in range(N, -1, -1):
    for j in range(i, -1, -1):
      for k in range(j, -1, - 1):
        if k == N: continue
        dp[i][j][k] = N
        dp[i][j][k] += (N - i) * dp[i + 1][j][k]
        dp[i][j][k] += (i - j) * dp[i][j + 1][k]
        dp[i][j][k] += (j - k) * dp[i][j][k + 1]
        dp[i][j][k] /= (N - k)

  ans = dp[a + b + c][b + c][c]
  print(f"{ans:.12f}")


if __name__ == "__main__":
  main()