#coding: utf-8
#yuki_366

n,k=map(int,raw_input().split())
a=map(int,raw_input().split())

b=a[:]
b.sort()
res=0

for i in range(k):
 for j in range(i, n, k):
  for l in range(i, j, k):
   if a[l] > a[j]:
    a[l], a[j] = a[j], a[l]
    res += 1

if a==b:
 print res
else:
 print -1