#include <bits/stdc++.h>

using namespace std;

typedef long long li;
const li INF64 = li(1e18);

int n, k;
const int N = 400043;
char buf[N];
string s;

li dp[N];
int pos_next[N];

int main()
{
	scanf("%d %d", &n, &k);
	scanf("%s", buf);
	s = buf;
	for(int i = 0; i < n; i++) dp[i] = INF64;
	dp[n] = 0;
	int cur = 2 * n;
	for(int i = 0; i < n; i++)
		pos_next[i] = N;
	for(int i = n - 1; i >= -k; i--)
	{
		if(i >= 0 && s[i] == '1')
			cur = i;
		if(i + k < n)
			pos_next[i + k] = cur;
	}
	for(int i = n; i > 0; i--)
	{
		if(pos_next[i - 1] - (i - 1) <= k)
		{
			int nw = max(0, pos_next[i - 1] - k);
			dp[nw] = min(dp[nw], dp[i] + pos_next[i - 1] + 1);
		}
		dp[i - 1] = min(dp[i - 1], dp[i] + i);
	}
	printf("%lld\n", dp[0]);
	return 0;
}