#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  Int n,k;
  cin>>n>>k;
  vector<Int> a(n);
  for(Int i=0;i<n;i++) cin>>a[i];
  Int ans=0;
  for(Int s=0;s<k;s++){
    for(Int i=s;i<n;i+=k)
      for(Int j=s;j<i;j+=k)
	ans+=a[j]>a[i];
      
    for(Int i=s;i<n;i+=k)
      for(Int j=s;j<i;j+=k)
	if(a[j]>a[i]) swap(a[i],a[j]);
  }
  if(!is_sorted(a.begin(),a.end())) ans=-1;
  cout<<ans<<endl;
  return 0;
}