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();//それぞれの値を受け取る
	}
	int Output = 0;//出力セット
	int MVal = N - K - 1;//MaxValue
	flag: for(int i = 0; i <= N; i++){
				for(int j = 0; j <= MVal; j++){
					if(0 > MVal){
						break flag;//0以下だと動かせないのでbreak
		}
		if(val[j] > val[j + K]){//ソート
			long memory = val[j];
			val[j] = val[j + K];
			val[j + K] = memory;
			Output++;
					}
			}
	}
	for(int i = 1; i < N; i++){//数字のソートの確認
		if(val[i - 1] > val[i]){
			Output = -1;
			break;
		}
	}
	System.out.println(Output);//結果を表示
}
}