#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n, k;
  cin >> n >> k;

  vector<int> a(n);
  REP (i, n) cin >> a[i];

  vector<int> b = a;
  sort(b.begin(), b.end());

  int ret = 0;

  REP (i, k) {
    for (int j = i; j < n; j += k) {
      int jt = j;
      while (jt - k >= 0 && a[jt - k] > a[jt]) {
        swap(a[jt], a[jt-k]);
        jt -= k;
        ++ret;
      }
    }
  }
  if (a != b)
    cout << -1 << endl;
  else
    cout << ret << endl;
  
  return 0;
}