using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;

namespace AtCoder
{
    static class Program_D
    {
        class Solver
        {
            
            public void Solve()
            {
                var nk = Ints();
                var n = nk[0];
                var k = nk[1];
                int[] p = Ints();
                float[] exp = new float[p.Length];
                for(int i=0;i<p.Length;++i)
                {
                    exp[i] = (float)((1 + p[i]) * p[i]) / (2 * p[i]);
                    //Console.WriteLine(string.Format("{0} : {1} : {2}",i,p[i],exp[i]));
                }

                double expSum=0;
                // setup
                for(int i =0; i < k; ++i)
                {
                    expSum += exp[i];
                }
                double expMax = expSum;
                // iteration
                for(int i=k; i <n;++i)
                {
                    var diff = exp[i] - exp[i - k];
                    expSum = expSum + diff;
                    //Console.WriteLine($"exp[{i}] - exp[{i - k}] = {diff}, expSum = {expSum}");
                    expMax = Math.Max(expMax, expSum);

                }
                Console.WriteLine(expMax);
            }



            // ----------------------------------------------------------------------------

            public int Int()
            {
                return int.Parse(Console.ReadLine());
            }
            public int[] Ints()
            {
                return Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToArray();
            }
            public long[] Longs()
            {
                return Console.ReadLine().Split(' ').Select(s => long.Parse(s)).ToArray();
            }
            public int[][] MultiInts(int num)
            {
                var result = new int[num][];
                for (int i = 0; i < num; ++i)
                {
                    result[i] = Ints();
                }
                return result;
            }
            public T[] MultiItems<T>(int num, Func<string, T> parser)
            {
                var result = new T[num];
                for (int i = 0; i < num; ++i)
                {
                    result[i] = parser(Console.ReadLine());
                }
                return result;
            }
        }



        static void Main(string[] args)
        {
            new Solver().Solve();
        }
    }
    struct ModInt
    {
        const int MOD = 1000000007;
        long Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ModInt(long data) { if ((Data = data % MOD) < 0) Data += MOD; }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator long(ModInt modInt) => modInt.Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator ModInt(long val) => new ModInt(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, ModInt b)
        {
            long res = a.Data + b.Data;
            return new ModInt() { Data = res >= MOD ? res - MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, long b) => a.Data + b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, ModInt b)
        {
            long res = a.Data - b.Data;
            return new ModInt() { Data = res < 0 ? res + MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, long b) => a.Data - b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator *(ModInt a, ModInt b) => new ModInt() { Data = a.Data * b.Data % MOD };
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator /(ModInt a, ModInt b) => a.Data * GetInverse(b);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override string ToString() => Data.ToString();

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        static long GetInverse(long a)
        {
            long div;
            long p = MOD;
            long x1 = 1, y1 = 0, x2 = 0, y2 = 1;
            while (true)
            {
                if (p == 1) return x2;
                div = a / p;
                x1 -= x2 * div;
                y1 -= y2 * div;
                a %= p;
                if (a == 1) return x1;
                div = p / a;
                x2 -= x1 * div;
                y2 -= y1 * div;
                p %= a;
            }
        }
    }

}


Measured memory_bytes: 31964