using System;
using System.Linq;
using System.Collections.Generic;
using Debug = System.Diagnostics.Debug;
using StringBuilder = System.Text.StringBuilder;
using System.Numerics;
using Point = System.Numerics.Complex;
using Number = System.Int64;
using C = System.Int32;
namespace Program
{
    public class Solver
    {
        public void Solve()
        {
            var s = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            s.Validate(x => x.Length == 2);
            var n = s[0].Validate(x => 2 <= x && x <= 100);
            var m = s[1].Validate(x => 1 <= x && x <= n);
            var A = new int[n + 1];
            if (n > 2)
            {
                s = Console.ReadLine().Split(' ').Select(int.Parse).ToArray().ValidateArray(x => 0 <= x && x <= 1000000000);
                s.Validate(x => x.Length == n - 2);
                for (int i = 2; i < n; i++)
                    A[i] = s[i - 2];
            }
            Debug.WriteLine(A.AsJoinedString());
            var dp = Enumerate(200, x => new double[n + 1]);
            for (int k = 0; k < 200; k++)
                for (int i = 0; i < n; i++)
                    dp[k][i] = -1;
            Func<int, int, double> dfs = null;
            dfs = (k, i) =>
              {
                  if (k < 0) return 0.0;
                  if (dp[k][i] > -0.5) return dp[k][i];
                  var ret = 0.0;
                  for (int j = 1; j <= m; j++)
                  {
                      var go = i + j;
                      if (go > n) go = n - (go - n);
                      ret += A[go];
                      ret += dfs(k - 1, go);
                  }
                  ret /= m;
                  return dp[k][i] = ret;
              };
            var DP = new double[n + 1];
            for (int i = 0; i <= n; i++)
                DP[i] = -1;
            Func<int, double> efs = null;
            efs = i =>
              {
                  //Debug.WriteLine(i);
                  if (DP[i] > -0.5) return DP[i];
                  if (i + m >= n) return 0;
                  var ret = 0.0;
                  var min = double.MaxValue;
                  for (int j = 1; j <= m; j++)
                  {
                      var go = i + j;
                      if (go > n) go = n - (go - n);
                      ret += A[go];
                      ret += efs(go);
                      min = Math.Min(min, A[go] + dfs(49, go));
                  }
                  ret /= m;



                  ret = Math.Min(ret, min);
                  return DP[i] = ret;
              };
            Console.WriteLine("{0:0.000000000}", efs(1));


        }

        static T[] Enumerate<T>(int n, Func<int, T> f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(i); return a; }
        static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
    }
}

#region main
static class Ex
{
    static public string AsString(this IEnumerable<char> ie) { return new string(System.Linq.Enumerable.ToArray(ie)); }
    static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ") { return string.Join(st, ie); }
    static public void Main()
    {
        var solver = new Program.Solver();
        solver.Solve();
    }
}
#endregion

static public class Validator
{
    static public T Validate<T>(this T input, Func<T, bool> f)
    {
        if (!f(input))
            throw new Exception("invalid input");
        return input;
    }
    static public T[] ValidateArray<T>(this T[] input, Func<T, bool> f)
    {
        foreach (var x in input)
            if (!f(x))
                throw new Exception("invalid input");
        return input;
    }
}