#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef long double ld;
typedef pair<int,int> P;

int main() {
  int n, k;
  cin >> n >> k;
  vector<int> a(n);

  rep(i,n) cin >> a[i];
  
  auto b = a;

  sort(b.begin(), b.end());
  map<int,int> id;

  for (int i = 0; i < n; i++) id[b[i]] = i;

  ll res = 0;

  for (int i = 0; i < n; i++) {

    if (id[a[i]] % k != i % k) {
      cout << -1 << endl;
      return 0;
    }

    for (int j = i; j >= 0; j -= k) if (a[j] > a[i]) res++;
  }

  cout << res << endl;
  
  return 0;
}