#include <bits/stdc++.h>
using namespace std;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FORR(x,arr) for(auto&& x:arr)
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define RFOR(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define ALL(x) (x).begin(), (x).end()
#define IV1(m,l,r) ((l) <= (m) && (m) < (r))
#define IV2(m,l,r) ((l) <= (m) && (m) <= (r))
#define BIT(n) (1LL<<(n))
#define SZ(x) ((int)(x).size())
typedef long long ll;
// -------------------------------------

int N, K;
int A[1000];

int bsort(int i) {
  int ret = 0;
  for (int j=i; j<N; j+=K) {
    for (int k=j+K; k<N; k+=K) {
      if (A[j] > A[k]) {
        ret++;
        swap(A[j], A[k]);
      }
    }
  }
  return ret;
}

void Main() {
  scanf("%d %d", &N, &K);
  REP(i, N) scanf("%d", A+i);

  int ans = 0;
  REP(i, K) {
    ans += bsort(i);
  }
  bool ok = true;
  REP(i, N-1) {
    if (A[i] > A[i+1]) {
      ok = false;
      break;
    }
  }
  if (ok) cout << ans << endl;
  else puts("-1");
}
int main() { cin.tie(0); ios::sync_with_stdio(false); Main(); return 0; }