#include <bits/extc++.h>

#include <atcoder/all>

#ifdef ONLINE_JUDGE

#include <bits/extc++.h>

#include <atcoder/all>

#else

// #include "all.h"

#endif

using ll = long long int;

template <class T>
bool chmin(T &x, const T val) {
  if (x > val) {
    x = val;
    return true;
  } else {
    return false;
  }
}

template <class T>
bool chmax(T &x, const T val) {
  if (x < val) {
    x = val;
    return true;
  } else {
    return false;
  }
}

template <class T, class U>
std::istream &operator>>(std::istream &is, std::pair<T, U> &p) {
  return is >> p.first >> p.second;
}

template <class T>
std::istream &operator>>(std::istream &is, std::vector<T> &v) {
  for (T &x : v) is >> x;
  return is;
}

template <class... T>
std::istream &operator>>(std::istream &is, std::tuple<T...> &tpl) {
  [&is, &tpl ]<size_t... I>(std::index_sequence<I...>) {
    (is >> ... >> std::get<I>(tpl));
  }
  (std::make_index_sequence<std::tuple_size_v<std::tuple<T...>>>{});
  return is;
}

template <class T>
std::ostream &operator<<(std::ostream &os, const std::vector<T> &v) {
  for (int i = 0; i < v.size(); i++)
    os << v[i] << (i == v.size() - 1 ? "" : " ");
  return os;
}

struct Initialization {
  Initialization() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
  }
} initialization;

constexpr std::pair<int, int> dir[] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};

template <typename T>
using infs = std::numeric_limits<T>;

template <typename T>
class factorials {
  size_t n;
  std::vector<T> fact, inv_fact;

  void extend(size_t m) {
    if (m <= n) return;
    fact.resize(m + 1);
    inv_fact.resize(m + 1);
    for (size_t i = n + 1; i <= m; i++) fact[i] = fact[i - 1] * i;
    inv_fact[m] = fact[m].inv();
    for (size_t i = m - 1; i >= n + 1; i--) inv_fact[i] = inv_fact[i + 1] * i;
    n = m;
  }

 public:
  factorials() : n(0), fact(1, 1), inv_fact(1, 1) {}

  factorials(size_t n) : n(n), fact(n + 1), inv_fact(n + 1) {
    fact[0] = 1;
    for (size_t i = 1; i <= n; i++) fact[i] = fact[i - 1] * i;
    inv_fact[n] = fact[n].inv();
    for (size_t i = n; i >= 1; i--) inv_fact[i - 1] = inv_fact[i] * i;
  }

  T inv(int k) {
    extend(k);
    return inv_fact[k];
  }

  T operator()(int k) {
    extend(k);
    return fact[k];
  }

  T perm(int n, int k) {
    if (n < k) return 0;
    if (k < 0) return 0;
    extend(n);
    return fact[n] * inv_fact[n - k];
  }

  T comb(int n, int k) {
    if (n < k) return 0;
    if (k < 0) return 0;
    extend(n);
    return fact[n] * inv_fact[n - k] * inv_fact[k];
  }
};

template <typename T>
class fps {
  std::vector<T> v;

 public:
  fps(int n) : v(n) {}
  fps(std::vector<T> v) : v(v) {}
};

using mint = atcoder::modint998244353;

int op(int a, int b) { return a + b; }

int e() { return 0; }

int main() {
  int n;
  ll m;
  std::cin >> n >> m;
  std::vector<ll> a(n);
  std::cin >> a;

  atcoder::segtree<int, op, e> seg(n);

  ll tento = 0;

  for (int i = 0; i < n; i++) {
    tento += seg.prod(a[i], n);
    seg.set(a[i] - 1, 1);
  }

  ll k = (tento + m - 1) / m * m;

  if (tento % 2 == 1 && m % 2 == 0) {
    std::cout << -1 << std::endl;
    return 0;
  }
  if (tento % 2 != k % 2) {
    k += m;
  }

  std::cout << k << std::endl;
}