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
{
    public void Solve()
    {
        int N = Reader.Int();
        int D = Reader.Int();
        var Cost = new int[N];
        for (int i = 1; i < N - 1; i++) Cost[i] = Reader.Int();
        Func<int, int, int> Next = (i, d) => N - 1 - Math.Abs(i + d - (N - 1));

        var M = new double[N - 1][];
        for (int i = 0; i < N - 1; i++)
        {
            M[i] = new double[N];
            M[i][i] = 1;
            for (int d = 1; d <= D; d++)
                M[i][Next(i, d)] -= 1.0 / D;
            M[i][N - 1] = Cost[i];
        }
        GaussJordan(M);
        var ans = new double[N, 2];
        for (int i = N - 2; i >= 0; i--)
        {
            double unuse = Cost[i], use = double.MaxValue;
            for (int d = 1; d <= D; d++)
            {
                if (i + d < N) use = Math.Min(use, Cost[i] + ans[i + d, 1]);
                unuse += ans[Next(i, d), 0] / D;
            }
            ans[i, 0] = Math.Min(unuse, use);
            ans[i, 1] = M[i][N - 1];
        }

        Console.WriteLine(ans[0, 0]);
    }

    int GaussJordan(double[][] M)
    {
        const double EPS = 1e-9;
        int H = M.Length;
        int W = M[0].Length;
        int rank = 0;
        for (int i = 0; i < H; i++)
        {
            int pivot = rank;
            for (int k = rank + 1; k < H; k++)
                if (Math.Abs(M[k][i]) > Math.Abs(M[pivot][i]))
                    pivot = k;
            var t = M[rank]; M[rank] = M[pivot]; M[pivot] = t;
            if (Math.Abs(M[rank][i]) < EPS) continue;
            for (int c = W - 1; c >= i; c--)
                M[rank][c] /= M[rank][i];
            for (int k = 0; k < H; k++)
                if (k != rank)
                    for (int c = W - 1; c >= i; c--)
                        M[k][c] -= M[k][i] * M[rank][c];
            rank++;
        }
        return rank;
    }
}


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;
    }
}