#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;
template<typename T, typename U>

pair<T, vector<U>> invNum(vector<U> a) {
  size_t n = a.size();
  if (n <= 1) return { (T) 0, a };
  size_t mid = n / 2;
  auto left = invNum<T, U>(vector<U>(a.begin(), a.begin() + mid));
  auto right = invNum<T, U>(vector<U>(a.begin() + mid, a.end()));
  vector<U> res;
  T inv = 0;
  inv += left.first + right.first;
  size_t i = 0, j = 0;
  while (i < left.second.size() && j < right.second.size()) {
    if (left.second[i] <= right.second[j]) {
      res.push_back(left.second[i]);
      i++;
    }
    else {
      res.push_back(right.second[j]);
      j++;
      inv += left.second.size() - i;
    }
  }
  while (i < left.second.size()) {
    res.push_back(left.second[i]);
    i++;
  }
  while (j < right.second.size()) {
    res.push_back(right.second[j]);
    j++;
  }
  return { inv, res };
}

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  rep(i, n) cin >> a[i];
  vector bs(k, vector<int>(0));
  rep(i, n) bs[i % k].push_back(a[i]);
  ll ans = 0;
  rep(i, k) {
    ans += invNum<int, int>(bs[i]).first;
    sort(bs[i].begin(), bs[i].end());
  }
  vector<int> b(n);
  rep(i, n) b[i] = bs[i % k][i / k];
  if (!is_sorted(b.begin(), b.end())) {
    cout << -1 << '\n';
  }
  else {
    cout << ans << '\n';
  }
  return 0;
}