#include <stdio.h>

int main(){
    char s[52];
    int n,k;
    int i;
    char *ps,*se;
    int cost,prize;

    scanf("%d %d %s",&n,&k,s);

    for(i=0;i<n;i++)s[i]-='0';

    cost=prize=0;
    ps=s;
    se=&s[n];
    for(i=k;i;i--){
        if(prize){
            prize--;
        }else{
            cost++;
        }
        prize+=*ps;
        if(++ps==se)ps=s;
    }
    printf("%d\n",cost);
    return 0;
}