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

namespace ABC124D
{
    class Program
    {
        static void Solve()
        {
            var n = Input.NextInt();
            var k = Input.NextInt();
            var s = Input.Next();

            // 奇数番地が白
            var list = new List<int>();
            var index = 0;
            while (index < s.Length)
            {
                var c0 = 0;
                var c1 = 0;
                while (index < s.Length && s[index] == '0')
                {
                    c0++;
                    index++;
                }
                while (index < s.Length && s[index] == '1')
                {
                    c1++;
                    index++;
                }

                list.Add(c0);
                list.Add(c1);
            }

            if (list.Count() <= 2 * k)
            {
                Console.WriteLine(list.Sum());
                return;
            }

            list.Add(0);

            // 末尾から埋めていく
            var slist = Enumerable.Repeat(0, list.Count()).ToList();
            slist[slist.Count() - (2 * k + 1)] = list.Skip(list.Count() - (2 * k + 1)).Sum();
            foreach (var i in Enumerable.Range(0, list.Count() - (2 * k + 1)).Reverse())
            {
                if ((i % 2) == 0)
                {
                    slist[i] = slist[i + 1]
                        + list[i]
                        - list[i + 2 * k] - list[i + 2 * k + 1];
                }
                else
                {
                    slist[i] = slist[i + 1] + list[i];
                }
            }

            var max = slist.Take(slist.Count() - k * 2).Max();
            Console.WriteLine(max);
        }

        #region Competitive Template
        public static void Main(string[] args)
        {
            var needsFlushOutput = true;
            if (needsFlushOutput)
            {
                // 細かく出力しないようにする
                var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
                Console.SetOut(sw);
            }

            // 仮想的に標準入力をセットする
            // NextLine系を使っていると使えない
            //Input.SetText("");
            //Input.SetText("");

            Solve();

            Console.Out.Flush();
        }

        static class Input
        {
            static char[] separator = { ' ' };
            public static bool IsEof { get; set; }
            static Queue<string> q { get; set; }

            static Input()
            {
                IsEof = false;
                q = new Queue<string>();
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="items"></param>
            public static void SetText(IEnumerable<string> items)
            {
                foreach (var item in items)
                {
                    SetText(item);
                }
            }

            /// <summary>
            /// 入力予約
            /// </summary>
            /// <param name="s"></param>
            /// <returns></returns>
            public static bool SetText(string s)
            {
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                return true;
            }

            /// <summary>
            /// 内部queueに入力からの値をsplitして格納する
            /// </summary>
            /// <returns></returns>
            static bool read()
            {
                var s = Console.ReadLine();
                if (s == null) return false;

                foreach (var elem in s.Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries))
                {
                    q.Enqueue(elem);
                }

                if (!q.Any()) return read();

                return true;
            }

            /// <summary>
            /// 次のstringを一つ読み込む
            /// </summary>
            /// <returns></returns>
            public static string Next()
            {
                if (!q.Any())
                {
                    if (!read())
                    {
                        IsEof = true;
                        return "";
                    }
                }

                return q.Dequeue();
            }

            public static int NextInt() => int.Parse(Next());
            public static long NextLong() => long.Parse(Next());
            public static double NextDouble() => double.Parse(Next());

            public static List<string> Next(int n) => Enumerable.Range(0, n).Select(_ => Next()).ToList();
            public static List<int> NextInt(int n) => Next(n).Select(x => int.Parse(x)).ToList();
            public static List<long> NextLong(int n) => Next(n).Select(x => long.Parse(x)).ToList();
            public static List<double> NextDouble(int n) => Next(n).Select(x => double.Parse(x)).ToList();

            public static List<string> NextLine() => Console.ReadLine().Trim().Split(separator, StringSplitOptions.RemoveEmptyEntries).ToList();
            public static List<int> NextIntLine() => NextLine().Select(x => int.Parse(x)).ToList();
            public static List<long> NextLongLine() => NextLine().Select(x => long.Parse(x)).ToList();
            public static List<double> NextDoubleLine() => NextLine().Select(x => double.Parse(x)).ToList();
        }
        #endregion
    }
}


Measured memory_bytes: 15200