#include <bits/stdc++.h>
typedef long long LL;
#define SORT(c) sort((c).begin(),(c).end())

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

using namespace std;
int main(void)
{
  int n,k;
  cin >> n >> k;
  vector<int> a;
  a.resize(n);
  REP(i,n) cin >> a[i];
  if(!n) return 0;
  int answer=0;
  REP(i,k){
    for(int j=i;j<n;j+=k)
      for(int l=i;l<j;l+=k)
	if(a[l]>a[j]){
	  ++answer;
	  swap(a[l],a[j]);
	}
  }
  REP(i,n-1)
    if(a[i]>a[i+1]){
      cout << -1 << endl;
      return 0;
    }
  cout << answer << endl;
  return 0;

}