import java.util.ArrayList;
import java.util.Scanner;

/**
 * Created by marcoaltran on 28/04/17.
 */
public class Solution {

    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int numCities = scan.nextInt();
        int towerRange = scan.nextInt();
        Boolean[] isTherePower = new Boolean[numCities];
        for (int i = 0; i < numCities; i++) {
            isTherePower[i] = scan.nextInt() != 0;
        }
        solveProblem(numCities, towerRange, isTherePower);

    }

    private static void solveProblem(int numCities, int towerRange, Boolean[] isTherePower) {
        for (int i = 1; i <= numCities; i++) {
            boolean solved = getSolution(i, numCities, towerRange, isTherePower,
                    new boolean[numCities], 0);
            if (solved) {
                return;
            }
        }
        System.out.println("-1");
    }

    private static boolean getSolution(int citiesToLight, int numCities, int towerRange,
                                    Boolean[] isTherePower, boolean[] solution, int index) {
        if(index == numCities){
            if (isASolution(solution, towerRange)) {
                System.out.println(citiesToLight);
                return true;
            }
            else{
                return false;
            }
        }
        boolean solved = getSolution(citiesToLight, numCities, towerRange, isTherePower, solution, index + 1);
        if(solved){
            return true;
        }
        if(isTherePower[index] && totalCitiesOnLight(solution.clone()) < citiesToLight){
            boolean[] newSolution = solution.clone();
            newSolution[index] = true;
            return getSolution(citiesToLight, numCities, towerRange, isTherePower, newSolution, index + 1);
        }
        return false;
    }

    private static int totalCitiesOnLight(boolean[] solution) {
        int citiesOnLight = 0;
        for (Boolean city : solution) {
            if(city){
                citiesOnLight++;
            }
        }
        return citiesOnLight;
    }

    private static boolean isASolution(boolean[] solution, int towerRange) {
        boolean[] citiesOnLight = new boolean[solution.length];
        for (int i = 0; i < solution.length; i++) {
            if(solution[i]){
                for (int j = 0; j < towerRange; j++) {
                    if (i + j < solution.length) {
                        citiesOnLight[i+j] = true;
                    }
                }
                for (int j = 0; j < towerRange; j++) {
                    if (i - j >= 0) {
                        citiesOnLight[i-j] = true;
                    }
                }
            }
        }
        for (Boolean cityOnLight : citiesOnLight) {
            if(!cityOnLight){
                return false;
            }
        }
        return true;
    }
}