package yuki_coder;

import java.util.Scanner;

public class No_366 {
public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int N = sc.nextInt();
	int K = sc.nextInt();
	long[] val = new long[N];
	for(int i = 0; i < N; i++){
		val[i] = sc.nextLong();
	}
	int c = 0;
	int MV = N - K - 1;
	flag: for(int i = 0; i <= MV; i++){
				for(int j = 0; j <= MV; j++){
					if(0 > MV){
						break flag;
		}
		if(val[j] > val[j + K]){
			long tmp = val[j];
			val[j] = val[j + K];
			val[j + K] = tmp;
			c++;
					}
			}
	}
	for(int i = 1; i < N; i++){
		if(val[i - 1] > val[i]){
			c = -1;
			break;
		}
	}
	System.out.println(c);
}
}