#include <bits/stdc++.h>
using namespace std;

int main() 
{
  int n, k, res = 0, i;
  cin >> n >> k;
  vector<int> a(n);
  for (int &x : a) cin >> x;
  //cin >> k;
  for(i = 0; i < n; i++)
      for (int j = i + k; j < n; j += k)
          if (a[i] > a[j]) 
            res++;
  for(i = 0; i < k; i++)
  {
      vector<int> v;
      for (int j = i; j < n; j += k) 
        v.push_back(a[j]);
      sort(v.begin(), v.end());
      for (int j = i, p = 0; j < n; j += k) 
        a[j] = v[p++];
  }
  for(i = 0; i < n - 1; i++)
      if (a[i] > a[i + 1]) 
        return cout << -1, 0;
  cout << res;
  return 0;
}