import java.util.*;

public class Exercises22{
  public static void main (String[] args){

    Scanner sc = new Scanner(System.in);

    int size = sc.nextInt();
    int armWidth = sc.nextInt();

    int[] array = new int[size];

    for (int n = 0; n < size; n++){
      array[n] = sc.nextInt();
    }

    int count = 0;
    boolean flag = true;

    for (int a = 0; a < armWidth; a++){
      for (int k = a; k < getLastIndex(a, size, armWidth); k += armWidth){
        for(int i = getLastIndex(a, size, armWidth); k < i; i -= armWidth){
          if (array[i] < array[i - armWidth]){
            int w = array[i];
            array[i] = array[i - armWidth];
            array[i - armWidth] = w;
            count++;
          }
        }
      }
    }

    for (int n = 1; n < size; n++){
      if (array[n - 1] <= array[n]){
        flag = true;

      }else{
        flag = false;
        break;
      }
    }

    if(flag == true){
      System.out.println(count);
    }else{
      System.out.println(-1);
    }
  }

  private static int getLastIndex(int a, int size, int armWidth){
    int extr = 0;
    if (size % armWidth > a){
      extr = 1;
    }
    return (size / armWidth - 1 + extr) * armWidth + a;
  }
}