/*
#include<kyopro/master>
void self::main(){
  LL(N);
  RD(lll,M);
  VL(N,a);--a;
  
  lll T=a.inversion();
  if(T%2){
    if(M%2){
      ll k=ceil(ceil(T,M)-1,2);
      pp(M*(2*k+1));
    }
    else{
      pp(-1);
    }
  }
  else{
    if(M%2){
      ll k=ceil(ceil(T,M),2);
      pp(M*(2*k));
    }
    else{
      ll k=ceil(T,M);
      pp(M*k);
    }
  }
}
*/

#include<bits/stdc++.h>
using namespace std;
namespace self{void main();void sub();template<ostream&o>void pp(const auto&...a);void epp(const auto&...a);}
int main(){cin.tie(nullptr)->sync_with_stdio(0);cout<<fixed<<setprecision(15);self::main();}
#define RD(T,...) T __VA_ARGS__;lin(__VA_ARGS__)
#define LL(...) RD(ll,__VA_ARGS__)
#define RDVL(T,n,...) vec<T>__VA_ARGS__;fe(refs(__VA_ARGS__),e)e.get().resizes(n);lin(__VA_ARGS__)
#define VL(n,...) RDVL(ll,n,__VA_ARGS__)
#define fe(a,e,...) for(auto&&__VA_OPT__([)e __VA_OPT__(,__VA_ARGS__]):a)
#define ef(a,e,...) for(auto&&__VA_OPT__([)e __VA_OPT__(,__VA_ARGS__]):ranges::reverse_view(a))
namespace self{
using ll=long long;
using lll=__int128_t;
using ulll=__uint128_t;
ostream&operator<<(ostream&o,const ulll&x){return(x<10?o:o<<x/10)<<ll(x%10);}
istream&operator>>(istream&i,lll&x){ll t;i>>t;x=t;return i;}
ostream&operator<<(ostream&o,const lll&x){return o<<(x<0?"-":"")<<ulll(x>0?x:-x);}
constexpr auto refs(auto&...a){return array{ref(a)...};}
void lin(auto&...a){(cin>>...>>a);}
auto ceil(auto x,auto y){if(y<0)x=-x,y=-y;return x<=0?x/y:(x-1)/y+1;}
template<class...A>using pack_back_t=tuple_element_t<sizeof...(A)-1,tuple<A...>>;
}
namespace self{
template<class T>class FenwickTree{
public:
  ll n_;
  vector<T>data_;
  FenwickTree(ll n):n_(n),data_(n){}
  void add(ll i,T x){
    assert(0<=i&&i<n_);
    i++;
    while(i<=n_){
      data_[i-1]+=x;
      i+=i&-i;
    }
  }
  T sum(ll l,ll r)const{
    assert(0<=l&&l<=r&&r<=n_);
    T res=0;
    while(r>0)res+=data_[r-1],r-=r&-r;
    while(l>0)res-=data_[l-1],l-=l&-l;
    return res;
  }
};
}
namespace self{
template<class V>istream&operator>>(istream&i,vector<V>&v){fe(v,e)i>>e;return i;}
template<class V>constexpr int depth=0;
template<class T>struct core_t_helper{using type=T;};
template<class T>using core_t=core_t_helper<T>::type;
template<class V>struct vec;
template<int D,class T>struct hvec_helper{using type=vec<typename hvec_helper<D-1,T>::type>;};
template<class T>struct hvec_helper<0,T>{using type=T;};
template<int D,class T>using hvec=hvec_helper<D,T>::type;
template<class V>struct vec:vector<V>{
  static constexpr int D=depth<V>+1;
  using C=core_t<V>;
  using vector<V>::vector;
  void resizes(const auto&...a){if constexpr(sizeof...(a)==D)*this=make(a...,C{});else{ }}
  static auto make(ll n,const auto&...a){
    if constexpr(sizeof...(a)==1)return vec<C>(n,array{a...}[0]);
    else { }
  }
  vec&operator--(){fe(*this,e)--e;return*this;}
  ll size()const{return vector<V>::size();}
  template<class F=less<>>vec stable_sort(F f={})const{vec v=*this;ranges::stable_sort(v,f);return v;}
  template<class F=less<>>auto sort_from(F f={})const{
    vec<ll>o(size());
    std::iota(o.begin(),o.end(),0);
    return o.stable_sort([&](ll i,ll j){return f((*this)[i],(*this)[j]);});
  }
  ll inversion()const{
    ll res=0;
    FenwickTree<ll>s(size());
    ef(sort_from(),i){
      res+=s.sum(0,i);
      s.add(i,1);
    }
    return res;
  }
};
template<class...A>requires(sizeof...(A)>=2)vec(const A&...a)->vec<hvec<sizeof...(A)-2,pack_back_t<A...>>>;
}
template<ostream&o=cout>void self::pp(const auto&...a){const char*c="";((o<<c<<a,c=" "),...);o<<'\n';}
void self::main(){
  LL(N);
  RD(lll,M);
  VL(N,a);--a;

  lll T=a.inversion();
  if(T%2){
    if(M%2){
      ll k=ceil(ceil(T,M)-1,2);
      pp(M*(2*k+1));
    }
    else{
      pp(-1);
    }
  }
  else{
    if(M%2){
      ll k=ceil(ceil(T,M),2);
      pp(M*(2*k));
    }
    else{
      ll k=ceil(T,M);
      pp(M*k);
    }
  }
}