using System;
using System.Collections.Generic;
using System.Linq;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;

namespace AtCoderProject
{
    struct Range
    {
        public int start;
        public int endExclusive;
    }
    public class Program
    {
        public object Calc()
        {
            var line = consoleReader.SplitInt32();
            var N = line[0];
            var K = line[1];

            var s = consoleReader.String();

            var chokuritsu = new List<Range>();

            var prev = '1';
            var start = 0;

            for (int i = 0; i < N; i++)
            {
                if (s[i] != prev)
                {
                    if (prev == '1')
                    {
                        start = i;
                    }
                    else
                    {
                        chokuritsu.Add(new Range { start = start, endExclusive = i });
                    }
                    prev = s[i];
                }
            }

            var count = chokuritsu.Count + (s[N - 1] == '0' ? 1 : 0);

            if (K >= count)
                return N;

            if (K == count - 1)
            {
                if (s[0] == '0' && s[N - 1] == '0')
                    return N - Math.Min(chokuritsu[0].endExclusive, N - start);
                if (s[0] == '0')
                    return N - chokuritsu[0].endExclusive;
                if (s[N - 1] == '0')
                    return start;
            }

            if (s[N - 1] == '0')
                chokuritsu.Add(new Range { start = start, endExclusive = N });
            var max = int.MinValue;
            for (int i = 0; i <= chokuritsu.Count - K; i++)
            {
                int rangeStart = i > 0 ? chokuritsu[i - 1].endExclusive : 0;
                int rangeEnd = i + K < chokuritsu.Count ? chokuritsu[i + K].start : N;
                int width = rangeEnd - rangeStart;
                if (max < width)
                    max = width;
            }

            return max;

        }


        const int mod = 1000000007;
        private readonly ConsoleReader consoleReader;
        public Program() : this(new ConsoleReader()) { }
        public Program(ConsoleReader consoleReader) { this.consoleReader = consoleReader; }
        static void Main() => Console.WriteLine(new Program().Calc());
        public static string MultiLine<T>(IEnumerable<T> source) => string.Join("\n", source);
    }

    public class ConsoleReader
    {
        public virtual string String() => Console.ReadLine();
        public int Int32() => int.Parse(String());
        public long Int64() => long.Parse(String());
        public ulong UInt64() => ulong.Parse(String());
        public double Double() => double.Parse(String());
        public string[] Split() => String().Split().ToArray();
        public int[] SplitInt32() => String().Split().Select(int.Parse).ToArray();
        public long[] SplitInt64() => String().Split().Select(long.Parse).ToArray();
        public ulong[] SplitUInt64() => String().Split().Select(ulong.Parse).ToArray();
        public double[] SplitDouble() => String().Split().Select(double.Parse).ToArray();

        public IEnumerable<string> Repeat(int count)
        {
            for (int i = 0; i < count; i++)
                yield return String();
        }
        public IEnumerable<int> RepeatInt32(int count)
        {
            for (int i = 0; i < count; i++)
                yield return Int32();
        }
        public IEnumerable<long> RepeatInt64(int count)
        {
            for (int i = 0; i < count; i++)
                yield return Int64();
        }
        public IEnumerable<string[]> RepeatSplit(int count)
        {
            for (int i = 0; i < count; i++)
                yield return Split();
        }
        public IEnumerable<int[]> RepeatSplitInt32(int count)
        {
            for (int i = 0; i < count; i++)
                yield return SplitInt32();
        }
    }
    static class DicExt
    {
        public static V GetOrDefault<K, V>(this Dictionary<K, V> dic, K key, V defaultValue = default(V))
        {
            V val;
            return dic.TryGetValue(key, out val) ? val : defaultValue;
        }
    }
}


Measured memory_bytes: 14432