content: import java.io.*;
  import java.math.*;
  import java.security.*;
  import java.text.*;
  import java.util.*;
  import java.util.concurrent.*;
  import java.util.regex.*;
  
  public class Solution {
  
    // Complete the pylons function below.
    static int pylons(int k, int[] arr) {
      int counter = 0;
      int index = 0;
      int notCovered = 1;
      int lastAvailable = -1;
      while (index < arr.length) {
        if (arr[index] == 1) {
          lastAvailable = index;
        }
  
        if (notCovered >= k) {
          if (lastAvailable == -1) {
            return -1;
          }
          notCovered = - (k - 1) + (index - lastAvailable) + 1;
          index++;
          counter++;
          lastAvailable = -1;
        } else if (index == arr.length - 1) {
          if (notCovered > 0) {
            if (lastAvailable != 0) {
              counter++;
            } else {
              return -1;
            }
          }
          index++;
        } else {
          notCovered++;
          index++;
        }
      }
      return counter;
    }
  
    private static final Scanner scanner = new Scanner(System.in);
  
    public static void main(String[] args) throws IOException {
      BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
  
      String[] nk = scanner.nextLine().split(" ");
  
      int n = Integer.parseInt(nk[0]);
  
      int k = Integer.parseInt(nk[1]);
  
      int[] arr = new int[n];
  
      String[] arrItems = scanner.nextLine().split(" ");
      scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");
  
      for (int i = 0; i < n; i++) {
        int arrItem = Integer.parseInt(arrItems[i]);
        arr[i] = arrItem;
      }
  
      int result = pylons(k, arr);
  
      bufferedWriter.write(String.valueOf(result));
      bufferedWriter.newLine();
  
      bufferedWriter.close();
  
      scanner.close();
    }
  }
  