package yukicoder;
import java.util.Arrays;
import java.util.Scanner;
public class Main{
	public static void main(String[] args)throws Exception{
		new Main().solve();
	}
	void solve(){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int k=sc.nextInt();
		long[] a=new long[n];
		for(int i=0;i<n;i++){
			a[i]=sc.nextLong();
		}
		int count=0;
		for(int i=0;i<n;i++){
			long min=999999999;
			int m=-1;
			for(int j=0;i+j*k<n;j++){
				if(min>a[i+j*k]){
					m=i+j*k;
					min=a[i+j*k];
				}
			}
			for(int j=0;m-(j+1)*k>=i;j++){
				long d=a[m-j*k];
				a[m-j*k]=a[m-(j+1)*k];
				a[m-(j+1)*k]=d;
				count++;
			}
		}
		for(int i=0;i+1<n;i++){
			if(a[i]>a[i+1]){
				System.out.println(-1);
				return;
			}
		}
		System.out.println(count);
		
	}
	void tr(Object...o){System.out.println(Arrays.deepToString(o));}

}