#!/usr/bin/env python3
#coding:utf-8

import math
from sys import stdin
# import numpy as np
# from matplotlib import pyplot as plt



def main():
	read=stdin.readline

	#edit here!
	n,k=map(int,read().split())
	a=list(map(int,read().split()))
	
	ans=0
	for i in range(0,k):
		v=[]
		for j in range(i,n,k):
			v.append(a[j])
		flag=True
		cnt=0
		j=0
		while flag:
			flag=False
			sz=len(v)
			for x in range(sz-1,j,-1):
				if v[x-1]>v[x]:
					v[x-1],v[x]=v[x],v[x-1]
					flag=True
					cnt+=1

			j+=1
		ans+=cnt
		for j in range(i,n,k):
			a[j]=v[j//k]
	ng=False
	for i in range(n-1):
		if a[i]>a[i+1]:
			ng=True
	if ng!=True:
		print(ans)
	else :
		print(-1)


if __name__ == '__main__':
    main()