import java.util.Arrays;
import java.util.Scanner;

public class Main {	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int N = sc.nextInt();
		int K = sc.nextInt();
		int[] a = new int[N];
		int[] sub = new int[N];
		
		for(int i=0; i<N; i++){
			a[i] = sc.nextInt();
			sub[i] = a[i];
		}

		Arrays.sort(sub);
		int cnt = Robo_sort(a,N ,K);
		
		if(!Arrays.equals(a, sub)){
			System.out.println("-1");
		}else{		
			System.out.println(cnt);
		}
	}
	public static int Robo_sort(int[] a, int N , int K){
		int count = 0;
		for(int i=0; i<N-K; i++){
			for(int j=0; j<N-K; j++){
				if(a[j] > a[j+K]){
					int temp = a[j];
					a[j] = a[j+K];
					a[j+K] = temp;
					count ++;
				}
			}
		}
		return count;
	}
}