using System;

public class Hello
{
    public static int buy, atari, eat;
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var k = int.Parse(line[1]);
        var s = Console.ReadLine().Trim();
        getAns(n, k, s);
    }
    static void getAns(int n, int k, string s)
    {
        var m = k / n;
        var mod = k % n;
        if (m== 0) { Console.WriteLine(sim0(n,k,s)); return; }
        if (mod == 0) m--;
        var a = 0;
        foreach (var x in s) a += x - '0';
        if (a > n) { Console.WriteLine(1); return; }
        var b = n - a;
        var nk = k - n * (m - 1);
        var ans2 = sim0(2 * n, nk, s + s);
        var ans = ans2 + b * (m - 1);
        Console.WriteLine(ans);
    }
    static int sim0(int n, int k, string s)
    {
        if (k == 1) return 1;
        buy = 1;
        atari += s[0] - '0';
        eat = 1;
        var p = 1;
        while (p < n)
        {
            if (atari > 0) atari--;
            else buy++;
            atari += s[p] - '0';
            eat++;
            if (eat == k) return buy;
            p++;
        }
        return -1;
    }
}