import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		final int K = sc.nextInt();
		
		int[] array = new int[N]; // 重複がないと楽で助かる.
		int[] sorted = new int[N];
		for(int i = 0; i < N; i++){
			array[i]  = sc.nextInt();
			sorted[i] = array[i];
		}
		
		Arrays.sort(sorted);
		
		int answer = 0;
		CONT:
		for(int i = 0; i < N; i++){
			for(int pos = i; pos < N; pos += K){
				if(array[pos] == sorted[i]){
					while(i != pos){
						answer++;
						{
							int tmp = array[pos];
							array[pos] = array[pos - K];
							array[pos - K] = tmp;
						}
						pos -= K;
					}
					
					continue CONT;
				}
			}
			
			answer = -1;
			break;
		}
		
		System.out.println(answer);
	}

}