#include <stack>
#include <cmath>
#include <vector>
#include <string.h>
#include <queue>
#include <stdio.h>
#include <iomanip>
#include <cstdio>
#include <algorithm>
#define int long long
#define ll long long
#define ls now << 1
#define rs now << 1 | 1
using namespace std;
const int N = 201010;
const int INF = 0x3f3f3f3f3f3f3f3f;
inline int read()
{
	int sum = 0, ff = 1;
	char c = getchar();
	while(c < '0' || c > '9')
	{
		if(c == '-')
		{
			ff = -1;
		}
		c = getchar();
	}
	while(c >= '0' && c <= '9')
	{
		sum = sum * 10 + c - '0';
		c = getchar();
	}
	return sum * ff;
}
int T, n, k;
char s[N];
int f[N][2];
int t[N << 2];
void insert(int now, int l, int r, int x, int g)
{
	if(l == r)
	{
		t[now] = g;
		return;
	}
	int mid = l + r >> 1;
	if(x <= mid)
	{
		insert(ls, l, mid, x, g);
	}
	else
	{
		insert(rs, mid + 1, r, x, g);
	}
	t[now] = min(t[ls], t[rs]);
}
int query(int now, int l, int r, int x, int y)
{
	if(x <= l && r <= y)
	{
		return t[now];
	}
	int mid = l + r >> 1;
	int res = INF;
	if(x <= mid)
	{
		res = min(res, query(ls, l, mid, x, y));
	}
	if(y > mid)
	{
		res = min(res, query(rs, mid + 1, r, x, y));
	}
	return res;
}
signed main()
{
	memset(t, 0x3f, sizeof(t));
	memset(f, 0x3f, sizeof(f));
	f[0][0] = f[0][1] = 0;
	n = read();
	k = read();
	scanf("%s", s);
	for(int i = 1; i <= n; i ++)
	{
		f[i][1] = min(f[i - 1][0], f[i - 1][1]) + i;
		if(i > 1)
		{
			f[i][0] = query(1, 1, n, max(i - k, (ll)1), i - 1);
		}
		if(s[i - 1] == '1')
		{
			f[i][1] = min(f[i][1], f[max((ll)0, i - k - 1)][0] + i);
			f[i][1] = min(f[i][1], f[max((ll)0, i - k - 1)][1] + i);
			insert(1, 1, n, i, f[i][1]);
		}
	}
	cout << min(f[n][1], f[n][0]);
	return 0;
}