using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Math;
using static System.Array;
using static AtCoder.Tool;
using static AtCoder.CalcL;
namespace AtCoder
{
    class AC
    {
        const int MOD = 1000000007;
        //const int MOD = 998244353;
        const int INF = int.MaxValue / 2;
        const long SINF = long.MaxValue / 2;
        const double EPS = 1e-8;
        static readonly int[] dI = { 0, 1, 0, -1 };
        static readonly int[] dJ = { 1, 0, -1, 0 };
        //static List<List<int>> G = new List<List<int>>();
        //static List<List<Edge>>G = new List<List<Edge>>();
        //static List<Edge> E = new List<Edge>();
        static void Main(string[] args)
        {
            //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            //Console.SetOut(sw);
            var cin = new Scanner();

            var input = cin.ReadSplitInt();
            int N = input[0];
            int K = input[1];
            var p = cin.ReadSplit_Double();
            var d = new double[N + 1];
            d[0] = 0;
            for(var i = 0; i < N; i++)
            {
                d[i + 1] = d[i] + ((p[i] + 1) / 2);
            }
            //Console.WriteLine(string.Join(",", d));
            double ans = 0;
            for (var i = K; i < N + 1; i++)
            {
                ans = Max(ans, d[i] - d[i - K]);
            }
            Console.WriteLine(ans);

            //Console.Out.Flush();
        }
        
        struct Edge
        {
            public int from;

            public int to;
            public long dist;
            public Edge(int t, long c)
            {
                from = -1;
                to = t;
                dist = c;
            }
            public Edge(int f, int t, long c)
            {
                from = f;
                to = t;
                dist = c;
            }

        }

        
    }
    
    public class Scanner
    {
        public int[] ReadSplitInt()
        {
            return ConvertAll(Console.ReadLine().Split(), int.Parse);
        }
        public long[] ReadSplitLong()
        {
            return ConvertAll(Console.ReadLine().Split(), long.Parse);
        }
        public double[] ReadSplit_Double()
        {
            return ConvertAll(Console.ReadLine().Split(), double.Parse);
        }
    }
    public static class Tool
    {
        static public void Initialize<T>(ref T[] array, T initialvalue)
        {
            for (var i = 0; i < array.Length; i++)
            {
                array[i] = initialvalue;
            }
        }
        static public void Swap<T>(ref T a, ref T b)
        {
            T keep = a;
            a = b;
            b = keep;
        }

        static public void Display<T>(T[,] array2d, int n, int m)
        {
            for (var i = 0; i < n; i++)
            {
                for (var j = 0; j < m; j++)
                {
                    Console.Write($"{array2d[i, j]} ");
                }
                Console.WriteLine();
            }
        }

        static public long LPow(int a, int b)
        {
            return (long)Pow(a, b);
        }
    }
    static public class CalcI
    {
        public static int Gcd(int a, int b)
        {
            if (a * b == 0) { return Max(a, b); }
            return a % b == 0 ? b : Gcd(b, a % b);
        }
        public static int Lcm(int a, int b)
        {
            int g = Gcd(a, b);
            return a / g * b;
        }
        public static int Ceil(int n, int div)
        {
            return (n + div - 1) / div;
        }
    }
    static public class CalcL
    {
        public static long Gcd(long a, long b)
        {
            if (a * b == 0) { return Max(a, b); }
            return a % b == 0 ? b : Gcd(b, a % b);
        }
        public static long Lcm(long a, long b)
        {
            long g = Gcd(a, b);
            return a / g * b;
        }
        public static long Ceil(long n, long div)
        {
            return (n + div - 1) / div;
        }
    }
    class Modulo
    {
        private const int M = 1000000007;
        private readonly int[] m_facs;
        public int Mul(int a, int b)
        {
            return (int)(Math.BigMul(a, b) % M);
        }
        public Modulo(int n)
        {
            m_facs = new int[n + 1];
            m_facs[0] = 1;
            for (int i = 1; i <= n; ++i)
            {
                m_facs[i] = Mul(m_facs[i - 1], i);
            }
        }
        public int Fac(int n)
        {
            return m_facs[n];
        }
        public int Pow(int a, int m)
        {
            switch (m)
            {
                case 0:
                    return 1;
                case 1:
                    return a;
                default:
                    int p1 = Pow(a, m / 2);
                    int p2 = Mul(p1, p1);
                    return ((m % 2) == 0) ? p2 : Mul(p2, a);
            }
        }
        public int Div(int a, int b)
        {
            return Mul(a, Pow(b, M - 2));
        }
        public int Ncr(int n, int r)
        {
            if (n < r) { return 0; }
            if (n == r) { return 1; }
            int res = Fac(n);
            res = Div(res, Fac(r));
            res = Div(res, Fac(n - r));
            return res;
        }
        public Modulo() { }
    }
}


Measured memory_bytes: 30556