secvente4

/* Secvente - O(N)
 * Florian Usurelu
 * scor: 100
 */
#include <bits/stdc++.h>
using namespace std;

ifstream f ("secvente.in");
ofstream g ("secvente.out");

const int nmax = 1e6 + 3;
char s[nmax];
int v[nmax], n, st[nmax], dr[nmax], k, sum, sol;
int main()
{
    f >> n >> k >> (s + 1);

    for (int i = 1; i <= n; ++i)
    {
        v[i] = s[i] - '0';
        sum += v[i];
    }

    if (sum < 2 * k)
    {
        g << -1;
        return 0;
    }

    sum = 0;

    for (int i = 1; i <= n; ++i)
    {
        sum += v[i];
        if(i - k >= 1)
        {
            sum -= v[i - k];
            st[i] = max(sum, st[i - 1]);
        }

        if(i == k) st[i] = sum;
    }

    sum = 0;

    for (int i = n; i >= 0; --i)
    {
        sum += v[i];
        if(i + k <= n)
        {
            sum -= v[i + k];
            dr[i] = max(sum, dr[i + 1]);
        }

        if(i == n - k + 1) dr[i] = sum;
    }

    sol = 2e9;

    for (int i = k; i + 1 <= n - k + 1; ++i)
        sol = min(sol, 2 * k - st[i] - dr[i + 1]);

    g << sol;

    return 0;
}