#if !__INCLUDE_LEVEL__

#include __FILE__
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/tag_and_trait.hpp>
using namespace __gnu_pbds;
template<typename T>
struct Tree : tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>{
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::order_of_key;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::find_by_order;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::insert;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::erase;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::end;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::lower_bound;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::upper_bound;

    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::size;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::begin;
    using tree<pair<T,int>, null_type, less<pair<T,int>>, rb_tree_tag, tree_order_statistics_node_update>::rbegin;

    int id = 0;
    bool erase1(T x){
        auto itr = lower_bound({x,-1});
        if(itr==end())return false;
        else if(itr->first!=x)return false;
        else{
            erase(itr);
            return true;
        }
    }
    void insert(T x){
        insert({x,id++});
    }
    T operator[](int k){
        assert(k<size());
        return find_by_order(k)->first;
    }
    T count(T k){
        return order_of_key({k+1,-1})-order_of_key({k,-1});
    }
    pair<int,bool> izyou_ord(T x){
        if(size()==0)return {-1,false};
        int res = order_of_key({x,-1});
        if(res==size())return {res,false};
        else return {res,true};
    }
    pair<int,bool> ika_ord(T x){
        if(size()==0)return {-1,false};
        int res = order_of_key({x,-1});
        res-=1;
        if(res==-1)return {res,false};
        else return {res,true};
    }
    T mn() const {
        assert(size());
        return begin()->first;
    }
    T mx() const {
        assert(size());
        return rbegin()->first;
    }
    pair<T, bool> ika(const T &a) const {
        if(size()==0)return {-1,false};
        if (a < mn()) return {-1, false};
        if (mx() <= a) return {mx(), true};
        auto itr = lower_bound({a,-1});
        if(itr->first == a)return {itr->first, true};
        else {
            itr--;
            return {itr->first, true};
        }
    }
    pair<T, bool> izyou(const T &a) {
        if(size()==0)return {-1,false};
        if (mx() < a) return {-1, false};
        return {lower_bound({a,-1})->first, true};
    }
};
//multiset可能なTree
//erase1…返り値がboolで消す値があったらtrue,なかったらfalse
//S[i]…i番目(昇順)の値(0-indexed)にアクセス可能
int main()
{
    ll N,M;cin >> N >> M;
    Tree<ll> S;
    ll Inv = 0;
    rep(i,N){
        ll P;cin >> P;
        int ord;
        bool ok;
        tie(ord,ok) = S.izyou_ord(P);
        if(!ok)ord = S.size();
        Inv += S.size() - ord;
        S.insert(P);
    }
    if(Inv==0){
        cout << 0 << "\n";
        exit(0);
    }
    ll K = 0;
    if(M<Inv)K = ceil(Inv,M);
    else if(Inv<=M)K = 1;
    ll km = K*M;
    if(M%2==1){//Mが奇数の場合
        if(abs(Inv-km)%2==1)cout << (K+1)*M << "\n";
        else cout << K*M << "\n";
    }
    else if(M%2==0){
        if(abs(Inv-km)%2==1)cout << -1 << "\n";
        else cout << K*M << "\n";
    }
    //cout << Inv << "\n";
    /*Inv=4,M=6の時間違い
    if(Inv%2 != M%2){
        cout << -1 << "\n";
        exit(0);
    }
    Inv <= k*M
    if(M%2==1){//Mが奇数の場合
        if(abs(Inv-M)%2==1)Add = 1;

        //abs(In)
    }
    
    */
}

#else
//tie(a, b, c) = t
//repをllにしているので時間ギリギリならintに変換
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i, n) for(long long int i = 0; i < n; i++)
#define rrep(i, n) for(long long int i = n-1; i >= 0; i--)
#define range(i, m, n) for(long long int i = m; i < n; i++)
#define fore(i,a) for(auto &i:a)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define Sum(v) accumulate(all(v),0LL)
#define minv(v) *min_element(all(v))
#define maxv(v) *max_element(all(v))
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
const ll INF = 1LL << 60;
const ll MOD1 = 1000000007;
const ll MOD2 = 998244353;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
ll SN(char s){return ll(s-'0');}
ll SN(string s){return stoll(s);}
int alpN(char s){return int(s-'a');}
int AlpN(char s){return int(s-'A');}
int Nalp(int n){return char(n+97);}
int NAlp(int n){return char(n+65);}
using mint = modint;
using mint1 = modint1000000007;
using mint2 = modint998244353;
using pll = pair<long long, long long>;
template <class T>ostream &operator<<(ostream &o,const vector<T>&v){for(int i=0;i<(int)v.size();i++)o<<(i>0?" ":"")<<v[i];return o;}//vector空白区切り出力
ostream& operator<<(ostream& os, const mint1& N) {return os << N.val();}//mint出力。デフォはmint1
ostream& operator<<(ostream& os, const mint2& N) {return os << N.val();}
template<class T> bool contain(const std::string& s, const T& v) {
    return s.find(v) != std::string::npos;
}
ll max(int x,ll y){return max((ll)x,y);}
ll max(ll x,int y){return max(x,(ll)y);}
ll min(int x,ll y){return min((ll)x,y);}
ll min(ll x,int y){return min(x,(ll)y);}
template <typename T>
struct edge {
    int src, to;
    T cost;
    edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
    edge& operator=(const int& x) {
        to = x;
        return *this;
    }
    operator int() const { return to; }
};

template <typename T>
using Edges = vector<edge<T> >;
template <typename T>
using WeightedGraph = vector<Edges<T> >;
using UnWeightedGraph = vector<vector<int> >;
template <typename T>
using Matrix = vector<vector<T> >;

//unorderd_mapの拡張…https://qiita.com/hamamu/items/4d081751b69aa3bb3557
template<class T> size_t HashCombine(const size_t seed,const T &v){
    return seed^(std::hash<T>()(v)+0x9e3779b9+(seed<<6)+(seed>>2));
}
/* pair用 */
template<class T,class S> struct std::hash<std::pair<T,S>>{
    size_t operator()(const std::pair<T,S> &keyval) const noexcept {
        return HashCombine(std::hash<T>()(keyval.first), keyval.second);
    }
};
/* vector用 */
template<class T> struct std::hash<std::vector<T>>{
    size_t operator()(const std::vector<T> &keyval) const noexcept {
        size_t s=0;
        for (auto&& v: keyval) s=HashCombine(s,v);
        return s;
    }
};
/* tuple用 */
template<int N> struct HashTupleCore{
    template<class Tuple> size_t operator()(const Tuple &keyval) const noexcept{
        size_t s=HashTupleCore<N-1>()(keyval);
        return HashCombine(s,std::get<N-1>(keyval));
    }
};
template <> struct HashTupleCore<0>{
    template<class Tuple> size_t operator()(const Tuple &keyval) const noexcept{ return 0; }
};
template<class... Args> struct std::hash<std::tuple<Args...>>{
    size_t operator()(const tuple<Args...> &keyval) const noexcept {
        return HashTupleCore<tuple_size<tuple<Args...>>::value>()(keyval);
    }
};
ll ceil(ll a, ll b){
  return (a + b - 1) / b;
}
struct string_converter {
    char start = 0;
    char type(const char &c) const { return (islower(c) ? 'a' : isupper(c) ? 'A' : isdigit(c) ? '0' : 0); }
    int convert(const char &c) {
        if(!start) start = type(c);
        return c - start;
    }
    int convert(const char &c, const string &chars) { return chars.find(c); }
    template <typename T> auto convert(const T &v) {
        vector<decltype(convert(v[0]))> ret;
        ret.reserve(v.size());
        for(auto &&e : v) ret.emplace_back(convert(e));
        return ret;
    }
    template <typename T> auto convert(const T &v, const string &chars) {
        vector<decltype(convert(v[0], chars))> ret;
        ret.reserve(v.size());
        for(auto &&e : v) ret.emplace_back(convert(e, chars));
        return ret;
    }
    int operator()(const char &v, char s = 0) {
        start = s;
        return convert(v);
    }
    int operator()(const char &v, const string &chars) { return convert(v, chars); }
    template <typename T> auto operator()(const T &v, char s = 0) {
        start = s;
        return convert(v);
    }
    template <typename T> auto operator()(const T &v, const string &chars) { return convert(v, chars); }
} toint;
#endif