#include <bits/stdc++.h>
 
using namespace std;
#ifndef ATCODER_FENWICKTREE_HPP
#define ATCODER_FENWICKTREE_HPP 1

#include <cassert>
#include <vector>

namespace atcoder {

// Reference: https://en.wikipedia.org/wiki/Fenwick_tree
template <class T> struct fenwick_tree {
    using U = unsigned;//internal::to_unsigned_t<T>;

  public:
    fenwick_tree() : _n(0) {}
    explicit fenwick_tree(int n) : _n(n), data(n) {}

    void add(int p, T x) {
        assert(0 <= p && p < _n);
        p++;
        while (p <= _n) {
            data[p - 1] += U(x);
            p += p & -p;
        }
    }

    T sum(int l, int r) {
        assert(0 <= l && l <= r && r <= _n);
        return sum(r) - sum(l);
    }

  private:
    int _n;
    std::vector<U> data;

    U sum(int r) {
        U s = 0;
        while (r > 0) {
            s += data[r - 1];
            r -= r & -r;
        }
        return s;
    }
};

}  // namespace atcoder

#endif  // ATCODER_FENWICKTREE_HPP

long long gcd(long long a, long long b) {
  if (a % b == 0) return b;
  return gcd(b, a%b);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int n;
  long long m;
  cin >> n >> m;
  vector<int> a(n);
  for (int i=0;i<n;++i) cin >> a[i];
  long long k {};
  {
    atcoder::fenwick_tree<int> fenwick_(n);
    for (int i=0;i<n;++i) {
      k += fenwick_.sum(a[i], n);
      fenwick_.add(a[i]-1, 1);
    }
  }
  // (k + 2* l) (l >= 0)
  if (k==0) {
    cout << k << '\n';
  } else if (k <= m && (m-k) % 2 == 0) {
    cout << m << '\n';
  } else if (m <= k) {
    long long g = gcd(m, k);
    // m*c >= k となる最小のp
    //  c >= (k/m)
    long long c = (k+m-1) / m;
    long long d = m * c;
    if ((d-k) % 2 == 0) {
      cout << d << '\n';
    } else {
      cout << "-1\n";
    }
  } else {
    cout << "-1\n";
  }
}