using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("8 4");
            WillReturn.Add("11101110");
            //0.857142857142857
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("8 4");
            WillReturn.Add("11011001");
            //0.833333333333333
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("10 4");
            WillReturn.Add("1001001001");
            //0.6
        }
        else if (InputPattern == "Input4") {
            return System.IO.File.ReadAllLines(@"D:\CSharp\TestConsole\test_in\in01.txt").ToList();
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static int mN;
    static int mK;

    static List<double> mAList = new List<double>();

    static void Main()
    {
        List<string> InputList = GetInputList();
        int[] wkArr = InputList[0].Split(' ').Select(pX => int.Parse(pX)).ToArray();
        mN = wkArr[0];
        mK = wkArr[1];

        string AStr = InputList[1];
        foreach (char EachA in AStr) {
            if (EachA == '0') mAList.Add(0);
            if (EachA == '1') mAList.Add(1);
        }

        // 全て1の場合
        if (mAList.TrueForAll(pX => pX == 1D)) {
            Console.WriteLine(1);
            return;
        }

        // 答えで二分探索
        double L = 0;
        double R = 1;

        while (L + 0.0000000001 < R) {
            double Mid = (L + R) / 2;

            bool Result = CanAchieve(Mid);
            if (Result) {
                L = Mid;
            }
            else {
                R = Mid;
            }
        }
        Console.WriteLine(R);
    }

    // NeedAVGを達成できるかを判定
    static bool CanAchieve(double pNeedAVG)
    {
        // 取得区間はK以上N以下なので、2倍の長さにしておく
        var CompareList = new List<double>();
        for (int I = 1; I <= 2; I++) {
            foreach (double EachA in mAList) {
                CompareList.Add(EachA - pNeedAVG);
            }
        }
        int UB = CompareList.Count - 1;

        double[] RunSumArr = new double[UB + 1];

        // 累積和を設定する
        for (int I = 0; I <= UB; I++) {
            RunSumArr[I] = CompareList[I];
            if (0 < I) {
                RunSumArr[I] += RunSumArr[I - 1];
            }
        }

        var InsDeque = new Deque<int>();

        // 終端を全探索
        bool FirstFlag = true;
        int PrevRangeEnd = -1;

        for (int I = 0; I <= UB; I++) {
            // 長さKの区間を取れなければcontinue
            if (I - mK + 1 < 0) continue;

            // 始点候補のSta
            int RangeSta = I - mN + 1;
            RangeSta = Math.Max(0, RangeSta);

            // 始点候補のEnd
            int RangeEnd = I - mK + 1;

            if (FirstFlag) {
                FirstFlag = false;
            }
            else {
                RangeSta = Math.Max(RangeSta, PrevRangeEnd + 1);
            }

            // 引退処理
            while (InsDeque.Count > 0) {
                int FrontInd = InsDeque[0];
                if (RangeSta <= FrontInd && FrontInd <= RangeEnd == false) {
                    InsDeque.PopFront();
                    continue;
                }
                break;
            }

            for (int J = RangeSta; J <= RangeEnd; J++) {
                // 押し出し処理
                while (InsDeque.Count > 0) {
                    int LastInd = InsDeque[InsDeque.Count - 1];
                    if (RunSumArr[LastInd] - CompareList[LastInd] > RunSumArr[J] - CompareList[J]) {
                        InsDeque.PopBack();
                        continue;
                    }
                    break;
                }
                // 追加処理
                InsDeque.PushBack(J);
            }
            PrevRangeEnd = RangeEnd;

            // 最小値
            double MinVal = RunSumArr[InsDeque[0]] - CompareList[InsDeque[0]];
            if (RunSumArr[I] - MinVal >= 0) return true;
        }
        return false;
    }
}

#region Deque
internal class Deque<T>
{
    T[] buf;
    int offset, count, capacity;

    internal int Count { get { return count; } }

    internal Deque(int cap) { buf = new T[capacity = cap]; }

    internal Deque() { buf = new T[capacity = 16]; }

    internal T this[int index]
    {
        get { return buf[GetIndex(index)]; }
        set { buf[GetIndex(index)] = value; }
    }

    private int GetIndex(int index)
    {
        if (index >= capacity)
            throw new IndexOutOfRangeException("out of range");
        int ret = index + offset;
        if (ret >= capacity)
            ret -= capacity;
        return ret;
    }

    internal void PushFront(T item)
    {
        if (count == capacity) Extend();
        if (--offset < 0) offset += buf.Length;
        buf[offset] = item;
        count++;
    }

    internal T PopFront()
    {
        if (count == 0)
            throw new InvalidOperationException("collection is empty");
        count--;
        T ret = buf[offset++];
        if (offset >= capacity) offset -= capacity;
        return ret;
    }

    internal void PushBack(T item)
    {
        if (count == capacity) Extend();
        int id = count + offset;
        count++;
        if (id >= capacity) id -= capacity;
        buf[id] = item;
    }

    internal T PopBack()
    {
        if (count == 0)
            throw new InvalidOperationException("collection is empty");
        return buf[GetIndex(--count)];
    }

    internal void Insert(int index, T item)
    {
        if (index > count) throw new IndexOutOfRangeException();
        this.PushFront(item);
        for (int i = 0; i < index; i++)
            this[i] = this[i + 1];
        this[index] = item;
    }

    internal T RemoveAt(int index)
    {
        if (index < 0 || index >= count) throw new IndexOutOfRangeException();
        T ret = this[index];
        for (int i = index; i > 0; i--)
            this[i] = this[i - 1];
        this.PopFront();
        return ret;
    }

    private void Extend()
    {
        T[] newBuffer = new T[capacity << 1];
        if (offset > capacity - count) {
            int len = buf.Length - offset;
            Array.Copy(buf, offset, newBuffer, 0, len);
            Array.Copy(buf, 0, newBuffer, len, count - len);
        }
        else Array.Copy(buf, offset, newBuffer, 0, count);
        buf = newBuffer;
        offset = 0;
        capacity <<= 1;
    }

    internal T[] Items//デバッグ時に中身を調べるためのプロパティ
    {
        get
        {
            T[] a = new T[count];
            for (int i = 0; i < count; i++)
                a[i] = this[i];
            return a;
        }
    }
}
#endregion