from functools import lru_cache @lru_cache(maxsize=None) def solve(k0, k1, k2, k3): if any([k < 0 for k in [k0, k1, k2]]): return -1 n = sum([k0, k1, k2, k3]) if (k0, k1, k2, k3) == (0, 0, 0, n): return 0 s = k3 / n s += (solve(k0 - 1, k1 + 1, k2, k3) + 1) * (k0 / n) s += (solve(k0, k1 - 1, k2 + 1, k3) + 1) * (k1 / n) s += (solve(k0, k1, k2 - 1, k3 + 1) + 1) * (k2 / n) s *= n / (n - k3) return s n = int(input()) a_ls = [int(i) for i in input().split()] k0 = a_ls.count(0) k1 = a_ls.count(1) k2 = a_ls.count(2) k3 = n - k0 - k1 - k2 e = solve(k0, k1, k2, k3) print(e)