#include <bits/stdc++.h>
using namespace std;
int f(int n, int d,string& no) 
{
    
    vector<int> j(n,INT_MAX);
    queue<int> q;
    j[0]=0;
    q.push(0);
    while (!q.empty()) 
    {
        int cur=q.front();
        q.pop();
        
        
        if (cur==n-1) 
        {
            return j[cur];
        }
        
        
        for (int i=1;i<=d;++i) 
        {
            int nxt=cur+i;
            if (nxt<n&&no[nxt]=='1'&&j[nxt]==INT_MAX) 
            {
                
                j[nxt]=j[cur]+1;
                
                q.push(nxt);
            }
        }
    }
    
    
    return -1;
}

int main() 
{
    int n,d;
    cin>>n>>d;
    string no;
    cin>>no;

    cout<<f(n,d,no)<<endl;

    return 0;
}