#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}



//2DBIT, 0-indexed;
template <typename T>
struct BIT2D{
  int H;
  T def;
  vector<ll> height;
  vector<vector<ll>> width;
  vector<vector<T>> data;
  function<T(T,T)> fx;
  BIT2D(vl &x,vl &y,T _def,function<T(T,T)> _fx){
    def=_def,fx=_fx;
    height=x;
    sort(all(height));
    height.erase(unique(all(height)),height.end());
    H=height.size();
    width.resize(H);
    data.resize(H);
    rep(i,x.size()){
      ll idx=lower_bound(all(height),x[i])-height.begin();
      width[idx].emplace_back(y[i]);
    }
    rep(i,H){
      sort(all(width[i]));
    }
    rep(i,H){
      width[i].erase(unique(all(width[i])),width[i].end());
      data[i].resize(width[i].size(),def);
      ll to=i|(i+1);
      if(to<H){
        vector<ll> hoge;
        merge(all(width[i]),all(width[to]),back_inserter(hoge));
        swap(width[to],hoge);
      }
    }
  }
  void change(ll _x,ll _y,T val){
    int a=lower_bound(all(height),_x)-height.begin();
    for(int x=a;x<H;x|=x+1){
      int b=lower_bound(all(width[x]),_y)-width[x].begin();
      for(int y=b;y<width[x].size();y|=y+1){
        data[x][y]=fx(data[x][y],val);
      }
    }
  }
  T fold(ll _x,ll _y){
    T ret=def;
    int a=lower_bound(all(height),_x)-height.begin();
    for(int x=a-1;x>=0;x=(x&(x+1))-1){
      int b=lower_bound(all(width[x]),_y)-width[x].begin();
      for(int y=b-1;y>=0;y=(y&(y+1))-1){
        ret=fx(ret,data[x][y]);
      }
    }
    return ret;
  }
};


//0-indexed,2冪のセグメントツリー
template <class T>
struct SegTree {
    private:
    int n;// 葉の数
    vector<T> data;// データを格納するvector
    T def; // 初期値かつ単位元
    function<T(T, T)> operation; // 区間クエリで使う処理
    function<T(T, T)> change;// 点更新で使う処理

    T find(int a, int b) {
        T val_left = def, val_right = def;
        for (a += (n - 1), b += (n - 1); a < b; a >>= 1, b >>= 1)
        {
            if ((a & 1) == 0){
                val_left = operation(val_left, data[a]);
            }
            if ((b & 1) == 0){
                val_right = operation(data[--b],val_right);
            }
        }
        return operation(val_left, val_right);
    }
    public:
    // _n:必要サイズ, _def:初期値かつ単位元, _operation:クエリ関数,
    // _change:更新関数
    SegTree(size_t _n, T _def, function<T(T, T)> _operation,
        function<T(T, T)> _change=[](T a,T b){return b;})
        : def(_def), operation(_operation), change(_change) {
        n = 1;
        while (n < _n) {
            n *= 2;
        }
        data = vector<T>(2 * n - 1, def);
    }
    void set(int i, T x) { data[i + n - 1] = x; }
    void build() {
        for (int k=n-2;k>=0;k--) data[k] = operation(data[2*k+1],data[2*k+2]);
    }
    // 場所i(0-indexed)の値をxで更新
    void update(int i, T x) {
        i += n - 1;
        data[i] = change(data[i], x);
        while (i > 0) {
            i = (i - 1) / 2;
            data[i] = operation(data[i * 2 + 1], data[i * 2 + 2]);
        }
    }
    T all_prod(){
      return data[0];
    }
    // [a, b)の区間クエリを実行
    T query(int a, int b) {
        //return _query(a, b, 0, 0, n);
        return find(a,b);
    }

    // 添字でアクセス
    T operator[](int i) {
        return data[i + n - 1];
    }
};
int main(){
    ll n,m;cin >> n >> m;
    vl v(n);rep(i,n)cin >> v[i],v[i]--;
    ll ans=0;
    SegTree<ll> st(n,0,[](ll a,ll b){return a+b;});
    rep(i,n){
        ans+=st.query(v[i],n);
        st.update(v[i],1);
    }
    ll q=(ans+m-1)/m*m;
    if((q-ans)%2==0){
        cout << q << endl;
    }
    else{
        q+=m;
        if((q-ans)%2==0)cout << q << endl;
        else cout << -1 << endl;
    }

}