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();
            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];
            s = Console.ReadLine().Split(' ').Select(int.Parse).ToArray().ValidateArray(x => 0 <= x && x <= 1000000000);
            for (int i = 2; i < n; i++)
                A[i] = s[i - 2];
            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(199, go));
                  }
                  ret /= m;



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


        }

        public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());
        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();
        Program.IO.Printer.Out.Flush();
    }
}
#endregion
#region Ex
namespace Program.IO
{
    using System.IO;
    using System.Text;
    using System.Globalization;
    public class Printer: StreamWriter
    {
        static Printer() { Out = new Printer(Console.OpenStandardOutput()) { AutoFlush = false }; }
        public static Printer Out { get; set; }
        public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
        public Printer(System.IO.Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
        public Printer(System.IO.Stream stream, Encoding encoding) : base(stream, encoding) { }
        public void Write<T>(string format, T[] source) { base.Write(format, source.OfType<object>().ToArray()); }
        public void WriteLine<T>(string format, T[] source) { base.WriteLine(format, source.OfType<object>().ToArray()); }
    }
    public class StreamScanner
    {
        public StreamScanner(Stream stream) { str = stream; }
        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }
        private byte read()
        {
            if (isEof) return 0;
            if (ptr >= len) { ptr = 0; if ((len = str.Read(buf, 0, 1024)) <= 0) { isEof = true; return 0; } }
            return buf[ptr++];
        }
        public char Char() { byte b = 0; do b = read(); while ((b < 33 || 126 < b) && !isEof); return (char)b; }

        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }
        public string ScanLine()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n'; b = (char)read())
                if (b == 0) break;
                else if (b != '\r') sb.Append(b);
            return sb.ToString();
        }
        public long Long()
        {
            if (isEof) return long.MinValue;
            long ret = 0; byte b = 0; var ng = false;
            do b = read();
            while (b != 0 && b != '-' && (b < '0' || '9' < b));
            if (b == 0) return long.MinValue;
            if (b == '-') { ng = true; b = read(); }
            for (; true; b = read())
            {
                if (b < '0' || '9' < b)
                    return ng ? -ret : ret;
                else ret = ret * 10 + b - '0';
            }
        }
        public int Integer() { return (isEof) ? int.MinValue : (int)Long(); }
        public double Double() { var s = Scan(); return s != "" ? double.Parse(s, CultureInfo.InvariantCulture) : double.NaN; }
        private T[] enumerate<T>(int n, Func<T> f)
        {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f();
            return a;
        }

        public char[] Char(int n) { return enumerate(n, Char); }
        public string[] Scan(int n) { return enumerate(n, Scan); }
        public double[] Double(int n) { return enumerate(n, Double); }
        public int[] Integer(int n) { return enumerate(n, Integer); }
        public long[] Long(int n) { return enumerate(n, Long); }
    }
}
#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;
    }
}