#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, K;
  cin >> N >> K;
  vector<int> a(N);
  for (int i = 0; i < N; i++){
    cin >> a[i];
  }
  vector<vector<int>> b(K);
  for (int i = 0; i < N; i++){
    b[i % K].push_back(a[i]);
  }
  int ans = 0;
  for (int i = 0; i < K; i++){
    int cnt = b[i].size();
    for (int j = 0; j < cnt; j++){
      for (int k = j + 1; k < cnt; k++){
        if (b[i][j] > b[i][k]){
          ans++;
        }
      }
    }
    sort(b[i].begin(), b[i].end());
  }
  for (int i = 0; i < N; i++){
    a[i] = b[i % K][i / K];
  }
  for (int i = 0; i < N - 1; i++){
    if (a[i] > a[i + 1]){
      cout << -1 << endl;
      return 0;
    }
  }
  cout << ans << endl;
}