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

public class Program
{
    static readonly double INF = 1e100;
    int N, D;
    int[] Cost;
    double?[,] ans;

    public void Solve()
    {
        N = Reader.Int();
        D = Reader.Int();
        Cost = new[] { 0 }.Concat(Reader.IntArray(N - 2)).Concat(new[] { 0 }).ToArray();
        ans = new double?[N, 2];
        Console.WriteLine(Rec(0, 0, 0));
    }

    private double Rec(int at, int used, int depth)
    {
        if (depth > N) return INF;
        if (at == N - 1) return 0;
        if (used == 0 && at + D >= N - 1) return Cost[at];
        if (ans[at, used].HasValue) return ans[at, used].Value;
        var nextProb = new double[N];

        for (int i = 1; i <= D; i++)
        {
            int next = at + i;
            if (next >= N) next = N - 2 - (next - N);
            nextProb[next] += 1.0 / D;
        }
        double res0 = Cost[at];
        double res1 = INF;
        for (int next = 0; next < N; next++)
            if (next != at && nextProb[next] > 0)
            {
                res0 += nextProb[next] * Rec(next, used, depth + 1);
                if (used == 0) res1 = Math.Min(res1, Rec(next, 1, depth + 1));
            }
        res0 /= 1 - nextProb[at];
        double res = Math.Min(res0, res1);

        ans[at, used] = res;
        return res;
    }
}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    static TextReader reader = Console.In;
    static readonly char[] separator = { ' ' };
    static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    static string[] A = new string[0];
    static int i;
    static void Init() { A = new string[0]; }
    public static void Set(TextReader r) { reader = r; Init(); }
    public static void Set(string file) { reader = new StreamReader(file); Init(); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Range(N, Int); }
    public static int[][] IntTable(int H) { return Range(H, IntLine); }
    public static string[] StringArray(int N) { return Range(N, Next); }
    public static string[][] StringTable(int N) { return Range(N, () => Split(Line())); }
    public static string Line() { return reader.ReadLine().Trim(); }
    public static T[] Range<T>(int N, Func<T> f) { var r = new T[N]; for (int i = 0; i < N; r[i++] = f()) ; return r; }
    static string[] Split(string s) { return s.Split(separator, op); }
    static string Next() { CheckNext(); return A[i++]; }
    static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}