/* -*- coding: utf-8 -*-
 *
 * 2382.cc:  No.2382 Amidakuji M - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef long long ll;

/* global variables */

int as[MAX_N], tmp[MAX_N];

/* subroutines */

ll merge_sort(int as[], int tmp[], int l, int r) { // [l, r)
  ll ic = 0;

  if (l + 1 < r) {
    int m = (l + r) / 2;
  
    ic = merge_sort(as, tmp, l, m) + merge_sort(as, tmp, m, r);

    int i = l, j = m, k = l;

    while (i < m && j < r) {
      if (as[i] <= as[j]) tmp[k++] = as[i++];
      else tmp[k++] = as[j++], ic += m - i;
    }

    while (i < m) tmp[k++] = as[i++];
    while (j < r) tmp[k++] = as[j++];

    for (int i = l; i < r; i++) as[i] = tmp[i];
  }  

  return ic;
}

/* main */

int main() {
  int n;
  ll m;
  scanf("%d%lld", &n, &m);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  ll c = merge_sort(as, tmp, 0, n);
  //printf("c=%lld, m=%lld\n", c, m);

  ll x = (c + m - 1) / m * m;
  if ((x - c) & 1) {
    x += m;
    if ((x - c) & 1) x = -1;
  }

  printf("%lld\n", x);

  return 0;
}