#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); ++i)

// Usage: T = 転倒数の型、U = 要素の型
template<typename T, typename U>
pair<T, vector<U>> invNum(vector<U> a) {
  size_t n = a.size();
  if (n <= 1) return { (T) 0, a };
  size_t mid = n / 2;
  auto left = invNum<T, U>(vector<U>(a.begin(), a.begin() + mid));
  auto right = invNum<T, U>(vector<U>(a.begin() + mid, a.end()));
  vector<U> res;
  T inv = 0;
  inv += left.first + right.first;
  size_t i = 0, j = 0;
  while (i < left.second.size() && j < right.second.size()) {
    if (left.second[i] <= right.second[j]) {
      res.push_back(left.second[i]);
      i++;
    }
    else {
      res.push_back(right.second[j]);
      j++;
      inv += left.second.size() - i;
    }
  }
  while (i < left.second.size()) {
    res.push_back(left.second[i]);
    i++;
  }
  while (j < right.second.size()) {
    res.push_back(right.second[j]);
    j++;
  }
  return { inv, res };
}

int main() {
  int n;
  ll m;
  cin >> n >> m;
  vector<int> p(n);
  rep(i, n) cin >> p[i];
  auto [inv, q] = invNum<ll, int>(p);
  Debug(inv);
  ll minim = (inv + m - 1) / m * m;
  if (minim % 2 == inv % 2) {
    cout << minim << endl;
  }
  else if ((minim + m) % 2 == inv % 2) {
    cout << minim + m << endl;
  }
  else {
    cout << -1 << endl;
  }
  return 0;
}