using System;
using System.IO;
using System.Text;
using System.Linq;
using System.Diagnostics;
using System.Collections.Generic;
using Enu = System.Linq.Enumerable;

class Program
{
    int N;
    double?[, ,] memo = new double?[101, 101, 101];
    int[] need;

    void Solve()
    {
        N = reader.Int();
        var A = reader.IntArray(N);
        need = new int[4];
        Array.ForEach(A, a => need[Math.Max(0, 3 - a)]++);
        var ans = Rec();
        Console.WriteLine(ans);
    }

    private double Rec()
    {
        if (memo[need[1], need[2], need[3]].HasValue) return memo[need[1], need[2], need[3]].Value;
        if (need[0] == N) return 0;
        double res = 1;
        double sumProb = 0;

        for (int x = 1; x <= 3; x++)
            if (need[x] > 0)
            {
                double p = (double)need[x] / N;
                sumProb += p;
                need[x]--; need[x - 1]++;
                res += p * Rec();
                need[x]++; need[x - 1]--;
            }
        res /= sumProb;
        memo[need[1], need[2], need[3]] = res;

        return res;
    }




    static void Main() { new Program().Solve(); }
    Reader reader = new Reader(Console.In);
    class Reader
    {
        private readonly TextReader reader;
        private readonly char[] separator = { ' ' };
        private readonly StringSplitOptions removeOp = StringSplitOptions.RemoveEmptyEntries;
        private string[] A = new string[0];
        private int i;

        public Reader(TextReader r) { reader = r; }
        public bool HasNext() { return Enqueue(); }
        public string String() { return Dequeue(); }
        public int Int() { return int.Parse(Dequeue()); }
        public long Long() { return long.Parse(Dequeue()); }
        public double Double() { return double.Parse(Dequeue()); }
        public int[] IntLine() { var s = Line(); return s == "" ? new int[0] : Array.ConvertAll(Split(s), int.Parse); }
        public int[] IntArray(int N) { return Enu.Range(0, N).Select(i => Int()).ToArray(); }
        public int[][] IntGrid(int H) { return Enu.Range(0, H).Select(i => IntLine()).ToArray(); }
        public string[] StringArray(int N) { return Enu.Range(0, N).Select(i => Line()).ToArray(); }
        public string Line() { return reader.ReadLine().Trim(); }
        private string[] Split(string s) { return s.Split(separator, removeOp); }
        private bool Enqueue()
        {
            if (i < A.Length) return true;
            string line = reader.ReadLine();
            if (line == null) return false;
            if (line == "") return Enqueue();
            A = Split(line);
            i = 0;
            return true;
        }
        private string Dequeue() { Enqueue(); return A[i++]; }
    }
}