#include <iostream>
#include <algorithm>
#include <vector>
#include <deque>
#include <queue>
#include <set>
#include <map>
#include <limits>
#include <cmath>
#include <iomanip>
#include <functional>
#include <random>
#include <set>
#include <atcoder/string>
#include <atcoder/math>
#include <unordered_map>
#include <climits>
using namespace std;
using ll = long long;
using namespace atcoder;

//zero indexed and vector
template< typename T>
class BIT
{
public:
    vector<T> data;
    BIT(long long n) : data(n) {}
    void add(int index, T v)
    {
        int N = data.size();
        for (int i = index; i < N; i |= i + 1)
        {
            data[i] += v;
        }
    }

    long long sum(int index)
    {
        T retValue = 0;
        for (int i = index; i >= 0; i = (i & (i + 1)) - 1)
        {
            retValue += data[i];
        }
        return retValue;
    }

    void show(){
        for (auto v:data){
            cout << v << endl;
        }
    }
};

// 0~N-1に注意,compressするか確認
ll inversion_number(vector<ll> &A){
    BIT<ll> bit(A.size()*2);
    ll ans = 0;
    for (int i = 0; i < A.size(); i++) {
        ans += i - bit.sum(A[i]);
        bit.add(A[i], 1);
    }
    return ans;
}

int solve()
{
  ll N,M;
  cin >> N >> M;
  vector<ll> P(N);
  for(int i = 0;i < N;i++){
    cin >> P[i];
    P[i]--;
  }
  ll k = inversion_number(P);
  ll ans1 = (k+M-1)/M*M;
  ll ans2 = (k+M-1)/M*M + M;
  if( (ans1 - k) % 2 == 0){
    cout << ans1 << endl;
  }else if((ans2- k) % 2 == 0){
    cout << ans2 << endl;
  }else{
    cout << -1 << endl;
  }
  return 0;
}

int main()
{
  // ll T;
  // cin >> T;
  // while(T--){
  solve();
  // }
  return 0;
}