import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        int k=sc.nextInt();
        int a[]=new int[n];
        for(int i=0;i<n;i++)
            a[i]=sc.nextInt();
        int start=0; int count=0;
        while(start<n){
        int j=k-1;
            if((start+j)>=n)
                j=n-start-1;
        while((start+j)>=0 && a[start+j]!=1 ){
            j--;
        }
           if(j>k || j<(-k+1) || (start+j)==(-1)){
              count=-1; break;}
        else{
        start=start+j+k;
            count++;}
        }
        System.out.println(count);
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    }
}