#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int ans = 0;
void Bsort(vector<int>::iterator f, vector<int>::iterator l){
  for(auto e = f; e != l; e++){
    for(auto i = l-1; i != e; i--){
      auto j = i;
      j--;
      if(*i < *j){
        iter_swap(i,j);
        ans++;
      }
    }
  }
}
int main(){
  int n,k;cin>>n>>k;
  vector<int> A[min(n,k)];
  for(int i = 0; n > i; i++){
    int t;cin>>t;
    A[i%k].push_back(t);
  }
  for(int i = 0; min(n,k) > i; i++){
    Bsort(A[i].begin(),A[i].end());
  }
  vector<int> B(n);
  for(int i = 0; n > i; i++){
    B[i] = A[i%k][i/k];
  }
  for(int i = 1; n > i; i++){
    if(B[i-1] > B[i]){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;

}