String Transformations - JAVA

// Author : Saurav Kalsoor
// String Transformations - JAVA


import java.util.*;

public class Test {

    static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) {
        int n = sc.nextInt();
        int k = sc.nextInt();

        String str = sc.next();


        System.out.println(stringTransformations(n, k, str));
    }

    public static int stringTransformations(int n, int k, String s){
        char[] str = s.toCharArray();
        int first = -1, last = -1;
        for(int i = 0;i < n; i++){
            if(str[i] == '1'){
                if(first == -1){
                    first = i;
                }else{
                    last = i;
                }
            }
        }

        if(first == -1){
            return 0;
        }else if(last == -1){
            return 1;
        }

        int count = 2;
        str[first] = '2';
        str[last] = '2';

        int prevChanged = first;

        while(prevChanged < last){

            if(last - prevChanged <= k)
                return count;

            int i = -1;
            for(int j = prevChanged+1; j <= Math.min(n - 1, (prevChanged + k)); j++){
                if(str[j] == '1'){
                    i = j;
                }
            }

            if(i == -1){
                return -1;
            }

            prevChanged = i;
            str[prevChanged] = '2';
            count++;

        }

        return count;
    }

}