#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main() {
  int n, k;
  int a[1000];
  int count = 0;
  cin >> n >> k;
  for (int ni = 0; ni < n; ++ni) {
    cin >> a[ni];
  }
  for (int d = 0; d < k; ++d) {
    vector<int> b;
    for (int i = d; i < n; i += k) {
      b.push_back(a[i]);
    }
    for (size_t i = 0; i < b.size(); ++i) {
      for (size_t j = i + 1; j < b.size(); ++j) {
        if (b[i] > b[j]) {
          ++count;
        }
      }
    }
    sort(b.begin(), b.end());
    for (size_t i = 0; i < b.size(); ++i) {
      a[d + k * i] = b[i];
    }
  }
  for (int ni = 0; ni < n - 1; ++ni) {
    if (a[ni] > a[ni + 1]) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << count << endl;
  return 0;
}