#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,n) for(int i = 0; i < (int)(n); ++i)
#define DEBUG(x) cerr << #x << " = " << x << endl

bool check(vector<int> A, vector<vector<int>> S, int K) {
  sort(A.begin(), A.end());
  REP(i,S.size()) {
    sort(S[i].begin(), S[i].end());
  }
  REP(i,A.size()) {
    if(A[i] != S[i % K][i / K]) return false;
  }
  return true;
}

int bubble_sort(const vector<int>& ary) {
  int ans = 0;
  REP(i,ary.size()) {
    REP(j,i) {
      if(ary[j] > ary[i]) ans++;
    }
  }
  return ans;
}

signed main() {
  int N, K; cin >> N >> K;
  vector<int> A(N);
  REP(i,N) cin >> A[i];
  vector<vector<int>> S(K);
  REP(i,N) {
    S[i % K].push_back(A[i]);
  }
  if(check(A, S, K) == false) {
    cout << -1 << endl;
  }
  else {
    int ans = 0;
    REP(i,K) ans += bubble_sort(S[i]);
    cout << ans << endl;
  }
}