import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static string[] s_rd;
T RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
string RDR()() { return readln.chomp; }
T[] ARR(T = long)(in string str, T fix = 0) { auto r = str.split.to!(T[]); r[] += fix; return r; }
size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
bool minimize(T)(ref T x, T y) { if (x > y) { x = y; return true; } else { return false; } }
bool maximize(T)(ref T x, T y) { if (x < y) { x = y; return true; } else { return false; } }

long mod = 10^^9 + 7;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

void main()
{	
	auto N = RD;
	auto K = RD;
	auto S = RD!string;

	long[] cnts;
	long cnt;
	char last = S[0];
	foreach (c; S)
	{
		if (c != last)
		{
			cnts ~= cnt;
			cnt = 1;
			last = c;
		}
		else
		{
			++cnt;
		}
	}
	cnts ~= cnt;

	long eo = S[0] == '0' ? 0 : 1;
	long len = cnts.length / 2;
	len += eo == 0 ? cnts.length % 2 : 0;

	auto dp = new long[](max(1, len-K+1));
	dp[0] = eo == 1 ? cnts[0] : 0;
	foreach (i; 0..K)
	{
		auto j = i * 2 + eo;
		if (j < cnts.length)
			dp[0] += cnts[j];
		if (j+1 < cnts.length)
			dp[0] += cnts[j+1];
	}

	long ans = dp[0];
	foreach (i; 1..dp.length)
	{
		dp[i] = dp[i-1];
		
		auto j = (i-1) * 2 + eo;
		dp[i] -= cnts[j];
		if (j >= 1)
			dp[i] -= cnts[j-1];

		auto k = (i+K-1) * 2 + eo;
		if (k < cnts.length)
			dp[i] += cnts[k];
		if (k+1 < cnts.length)
			dp[i] += cnts[k+1];

		ans = max(ans, dp[i]);
	}

	debug writeln(cnts);
	debug writeln(dp);
	
	writeln(ans);
	stdout.flush();
	debug readln();
}