#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
int main()
{
    char in[10005];
    int n,m;
    scanf("%d%d",&n,&m);
    scanf("%s",&in);
    in[n]='0';
    int count=0;
    int start=0;
    int end=start;
    while(end<=n)
    {
        while(in[end]=='1'&&end<=n)
        {
            end++;
        }
        
        if(end-start==m)count++;
        else if(end-start>m)
        {
            int ans=end-start-m+1;
            while(ans)
            {
                count+=ans;
                ans--;
            }
        }

        start=end+1;
        end=start;


    }
    printf("%d",count);


    return 0;
}