import java.util.Scanner;


public class Main_yukicoder78 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int k = sc.nextInt();
        char[] s = sc.next().toCharArray();
        int[] a = new int[n + 1];
        int[] min = new int[n + 1];

        for (int i = 1; i <= n; i++) {
        	if (a[i - 1] > 0) {
        		min[i] = min[i - 1];
        		a[i] = a[i - 1] - 1 + s[i - 1] - '0';
        	} else {
        		min[i] = min[i - 1] + 1;
        		a[i] = a[i - 1] + s[i - 1] - '0';
        	}
        }
        
        if (k > n && a[n] >= min[n]) {
        	System.out.println(min[n]);
        } else if (k <= n) {
        	System.out.println(min[k]);
        } else {
        	long ret = (min[n] - a[n]) * (k / n - 1);
        	ret += min[n] + Math.max(0, min[k % n] - a[n]);
        	
        	System.out.println(ret);
        }

        sc.close();
    }
}