using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using static System.Math;
using System.Text;
using System.Threading;
using System.Globalization;
using System.Runtime.CompilerServices;
using Library;

namespace Program
{
    public static class ProblemD
    {
        static bool SAIKI = false;
        static public int numberOfRandomCases = 0;
        static public void MakeTestCase(List<string> _input, List<string> _output, ref Func<string[], bool> _outputChecker)
        {
        }
        static public void Solve()
        {
            var N = NN;
            var K = NN;
            var pList = NNList(N);
            var swag = new LIB_SlidingWindowAggregation<double>((x, y) => x + y);
            var kitaitiList = pList.Select(e => e * (e + 1) / 2 / (double)e).ToArray();
            for (var i = 0; i < K - 1; i++)
            {
                swag.PushBack(kitaitiList[i]);
            }
            var ans = 0.0;
            for (var i = K - 1; i < N; i++)
            {
                swag.PushBack(kitaitiList[i]);
                ans = Max(ans, swag.Aggregate());
                swag.PopFront();
            }
            Console.WriteLine(ans);
        }
        class Printer : StreamWriter
        {
            public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
            public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { base.AutoFlush = false; }
            public Printer(Stream stream, Encoding encoding) : base(stream, encoding) { base.AutoFlush = false; }
        }
        static LIB_FastIO fastio = new LIB_FastIODebug();
        static public void Main(string[] args) { if (args.Length == 0) { fastio = new LIB_FastIO(); Console.SetOut(new Printer(Console.OpenStandardOutput())); } if (SAIKI) { var t = new Thread(Solve, 134217728); t.Start(); t.Join(); } else Solve(); Console.Out.Flush(); }
        static long NN => fastio.Long();
        static double ND => fastio.Double();
        static string NS => fastio.Scan();
        static long[] NNList(long N) => Repeat(0, N).Select(_ => NN).ToArray();
        static double[] NDList(long N) => Repeat(0, N).Select(_ => ND).ToArray();
        static string[] NSList(long N) => Repeat(0, N).Select(_ => NS).ToArray();
        static long Count<T>(this IEnumerable<T> x, Func<T, bool> pred) => Enumerable.Count(x, pred);
        static IEnumerable<T> Repeat<T>(T v, long n) => Enumerable.Repeat<T>(v, (int)n);
        static IEnumerable<int> Range(long s, long c) => Enumerable.Range((int)s, (int)c);
        static IOrderedEnumerable<T> OrderByRand<T>(this IEnumerable<T> x) => Enumerable.OrderBy(x, _ => xorshift);
        static IOrderedEnumerable<T> OrderBy<T>(this IEnumerable<T> x) => Enumerable.OrderBy(x.OrderByRand(), e => e);
        static IOrderedEnumerable<T1> OrderBy<T1, T2>(this IEnumerable<T1> x, Func<T1, T2> selector) => Enumerable.OrderBy(x.OrderByRand(), selector);
        static IOrderedEnumerable<T> OrderByDescending<T>(this IEnumerable<T> x) => Enumerable.OrderByDescending(x.OrderByRand(), e => e);
        static IOrderedEnumerable<T1> OrderByDescending<T1, T2>(this IEnumerable<T1> x, Func<T1, T2> selector) => Enumerable.OrderByDescending(x.OrderByRand(), selector);
        static IOrderedEnumerable<string> OrderBy(this IEnumerable<string> x) => x.OrderByRand().OrderBy(e => e, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<T> OrderBy<T>(this IEnumerable<T> x, Func<T, string> selector) => x.OrderByRand().OrderBy(selector, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<string> OrderByDescending(this IEnumerable<string> x) => x.OrderByRand().OrderByDescending(e => e, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<T> OrderByDescending<T>(this IEnumerable<T> x, Func<T, string> selector) => x.OrderByRand().OrderByDescending(selector, StringComparer.OrdinalIgnoreCase);
        static uint xorshift { get { _xsi.MoveNext(); return _xsi.Current; } }
        static IEnumerator<uint> _xsi = _xsc();
        static IEnumerator<uint> _xsc() { uint x = 123456789, y = 362436069, z = 521288629, w = (uint)(DateTime.Now.Ticks & 0xffffffff); while (true) { var t = x ^ (x << 11); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); yield return w; } }
    }
}
namespace Library {
    class LIB_FastIO
    {
        public LIB_FastIO() { str = Console.OpenStandardInput(); }
        readonly Stream str;
        readonly byte[] buf = new byte[1024];
        int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }
        byte read()
        {
            if (isEof) throw new EndOfStreamException();
            if (ptr >= len)
            {
                ptr = 0;
                if ((len = str.Read(buf, 0, 1024)) <= 0)
                {
                    isEof = true;
                    return 0;
                }
            }
            return buf[ptr++];
        }
        char Char()
        {
            byte b = 0;
            do b = read();
            while (b < 33 || 126 < b);
            return (char)b;
        }
        virtual public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }
        virtual public long Long()
        {
            long ret = 0; byte b = 0; var ng = false;
            do b = read();
            while (b != '-' && (b < '0' || '9' < b));
            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';
            }
        }
        virtual public double Double() { return double.Parse(Scan(), CultureInfo.InvariantCulture); }
    }
    class LIB_FastIODebug : LIB_FastIO
    {
        Queue<string> param = new Queue<string>();
        string NextString() { if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item); return param.Dequeue(); }
        public LIB_FastIODebug() { }
        public override string Scan() => NextString();
        public override long Long() => long.Parse(NextString());
        public override double Double() => double.Parse(NextString());
    }
    class LIB_SlidingWindowAggregation<T>
    {
        T[][] list;
        T[][] agg;
        int[] size;
        Func<T, T, T> fun;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_SlidingWindowAggregation(IEnumerable<T> ary, Func<T, T, T> f)
        {
            list = new T[2][]; agg = new T[2][]; size = new int[2];
            fun = f;
            list[0] = new T[16]; agg[0] = new T[16]; size[0] = 0;
            if (ary.Any())
            {
                var temp = ary.ToArray();
                size[1] = temp.Length;
                list[1] = new T[size[1]];
                agg[1] = new T[size[1]];
                for (var i = 0; i < list[1].Length; i++)
                {
                    list[1][i] = temp[i];
                    if (i == 0) agg[1][i] = temp[i];
                    else agg[1][i] = fun(agg[1][i - 1], temp[i]);
                }
            }
            else
            {
                list[1] = new T[16]; agg[1] = new T[16]; size[1] = 0;
            }
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public LIB_SlidingWindowAggregation(Func<T, T, T> f) : this(new T[0], f) { }
        public int Count => size[0] + size[1];
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Clear()
        {
            size[0] = size[1] = 0;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        void Push(int lista, T val)
        {
            if (list[lista].Length == size[lista])
            {
                var newAry = new T[size[lista] * 2];
                var newAgg = new T[size[lista] * 2];
                Array.Copy(list[lista], newAry, size[lista]);
                Array.Copy(agg[lista], newAgg, size[lista]);
                list[lista] = newAry;
                agg[lista] = newAgg;
            }
            if (size[lista] == 0) agg[lista][0] = val;
            else if (lista == 0) agg[lista][size[lista]] = fun(val, agg[lista][size[lista] - 1]);
            else agg[lista][size[lista]] = fun(agg[lista][size[lista] - 1], val);
            list[lista][size[lista]++] = val;
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void PushBack(T val) => Push(1, val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void PushFront(T val) => Push(0, val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        T Pop(int lista)
        {
            var listb = 1 - lista;
            if (size[lista] == 0)
            {
                if (size[listb] == 0) throw new Exception();
                var mid = (size[listb] - 1) / 2;
                if (list[lista].Length <= mid)
                {
                    list[lista] = new T[mid + 1];
                    agg[lista] = new T[mid + 1];
                }
                size[lista] = 0;
                for (var i = mid; i >= 0; i--)
                {
                    if (size[lista] == 0) agg[lista][size[lista]] = list[listb][i];
                    else if (lista == 0) agg[lista][size[lista]] = fun(list[listb][i], agg[lista][size[lista] - 1]);
                    else agg[lista][size[lista]] = fun(agg[lista][size[lista] - 1], list[listb][i]);
                    list[lista][size[lista]++] = list[listb][i];
                }
                for (var i = mid + 1; i < size[listb]; i++)
                {
                    var idx = i - mid - 1;
                    if (idx == 0) agg[listb][idx] = list[listb][i];
                    else if (lista == 0) agg[listb][idx] = fun(list[listb][i], agg[listb][idx - 1]);
                    else agg[listb][idx] = fun(agg[listb][idx - 1], list[listb][i]);
                    list[listb][idx] = list[listb][i];
                }
                size[listb] -= size[lista];
            }
            return list[lista][--size[lista]];
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T PopBack() => Pop(1);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T PopFront() => Pop(0);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Aggregate()
        {
            if (size[0] == 0 && size[1] == 0) throw new Exception();
            else if (size[1] == 0) return agg[0][size[0] - 1];
            else if (size[0] == 0) return agg[1][size[1] - 1];
            else return fun(agg[0][size[0] - 1], agg[1][size[1] - 1]);
        }
    }
}


Measured memory_bytes: 28384