#ifdef DEBUG
#include"stdlibrary.h"
#else
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
using namespace std;
#endif
// #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;
// #include<boost/multiprecision/cpp_int.hpp>
// namespace multiprecisioninteger = boost::multiprecision;
// using cint=multiprecisioninteger::cpp_int;
using ll=long long;
using datas=std::pair<ll,ll>;
using ddatas=std::pair<long double,long double>;
using tdata=std::pair<ll,datas>;
using vec=std::vector<ll>;
using mat=std::vector<vec>;
using pvec=std::vector<datas>;
using pmat=std::vector<pvec>;
// using llset=tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>;
#define For(i,a,b) for(i=a;i<(ll)b;++i)
#define bFor(i,b,a) for(i=b,--i;i>=(ll)a;--i)
#define rep(i,N) For(i,0,N)
#define rep1(i,N) For(i,1,N)
#define brep(i,N) bFor(i,N,0)
#define brep1(i,N) bFor(i,N,1)
#define all(v) std::begin(v),std::end(v)
#define allr(v) std::rbegin(v),std::rend(v)
#define vsort(v) std::sort(all(v))
#define vrsort(v) std::sort(allr(v))
#define uniq(v) vsort(v),(v).erase(std::unique(all(v)),std::end(v))
#define endl "\n"
#define popcount __builtin_popcountll
#define print(x) std::cout<<x<<endl
#define printyes print("Yes")
#define printno print("No")
#define printYES print("YES")
#define printNO print("NO")
#define output(v) do{bool f=0;for(auto outi:v){std::cout<<(f?" ":"")<<outi;f=1;}std::cout<<endl;}while(0)
#define matoutput(v) do{for(auto outimat:v)output(outimat);}while(0)
// constexpr ll mod=1000000007;
constexpr ll mod=998244353;
constexpr ll inf=1LL<<60;
constexpr long double eps=1e-9;
const long double PI=acosl(-1);
template<class T,auto x=T::mod()> std::ostream& operator<<(std::ostream& os,const T& v){return os<<v.val();}
template<class T> std::ostream& operator<<(std::ostream& os,const std::vector<T>& v);
template<class T,class F> std::ostream& operator<<(std::ostream& os,const std::set<T,F>& v);
template<class T,class F> std::ostream& operator<<(std::ostream& os,const std::multiset<T,F>& v);
template<class T,class E,class F> std::ostream& operator<<(std::ostream& os,const std::map<T,E,F>& v);
template<class T,class E> std::ostream& operator<<(std::ostream& os,const std::pair<T,E>& p){return os<<"("<<p.first<<","<<p.second<<")";}
template<class T> std::ostream& operator<<(std::ostream& os,const std::vector<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T,class F> std::ostream& operator<<(std::ostream& os,const std::set<T,F>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T,class F> std::ostream& operator<<(std::ostream& os,const std::multiset<T,F>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T,class E,class F> std::ostream& operator<<(std::ostream& os,const std::map<T,E,F>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> inline bool chmax(T& a,const T b){bool x=a<b;if(x)a=b;return x;}
template<class T> inline bool chmin(T& a,const T b){bool x=a>b;if(x)a=b;return x;}
#ifdef DEBUG
void debugg(){std::cout<<endl;}
template<class T,class... Args>void debugg(const T& x,const Args&... args){std::cout<<" "<<x;debugg(args...);}
#define debug(...) cout<<__LINE__<<" ["<<#__VA_ARGS__<<"]:",debugg(__VA_ARGS__)
#else
#define debug(...) (void(0))
#endif

inline void startupcpp(void) noexcept{
  cin.tie(0)->sync_with_stdio(0);
  cout<<fixed<<setprecision(15);
}

ll modinv(ll a,const ll m=mod) noexcept{
  ll b=m,u=1,v=0,t;
  while(b){
    t=a/b;
    a-=t*b; swap(a,b);
    u-=t*v; swap(u,v);
  }
  return (u+m)%m;
}

ll moddevide(const ll a,const ll b) noexcept{return (a*modinv(b))%mod;}

vec modncrlistp,modncrlistm;

ll modncr(const ll n,const ll r){
  if(n<r||r<0)return 0;
  ll i,size=modncrlistp.size();
  if(size<=n){
    modncrlistp.resize(n+1);
    modncrlistm.resize(n+1);
    if(!size){
      modncrlistp[0]=modncrlistm[0]=1;
      size++;
    }
    For(i,size,n+1)modncrlistp[i]=modncrlistp[i-1]*i%mod;
    modncrlistm[n]=modinv(modncrlistp[n]);
    for(i=n;i>size;--i)modncrlistm[i-1]=modncrlistm[i]*i%mod;
  }
  return modncrlistp[n]*modncrlistm[r]%mod*modncrlistm[n-r]%mod;
}

ll modpow(ll a,ll n,const ll m=mod){
  if(n<0)return 0;
  ll res=1;
  while(n>0){
    if(n&1)res=res*a%m;
    a=a*a%m;
    n>>=1;
  }
  return res;
}

constexpr ll gcd(const ll a,const ll b) noexcept{return (!b)?abs(a):(a%b==0)?abs(b):gcd(b,a%b);}
constexpr ll lcm(const ll a,const ll b) noexcept{return a/gcd(a,b)*b;}

#include<atcoder/all>
using mint=atcoder::modint998244353;
// 転倒数を計算する
template<typename Iterator, bool is_permutation = false>
long long calcurate_inversion_number(const Iterator &_begin, const Iterator &_end) {
    auto N = std::distance(_begin, _end);
    atcoder::fenwick_tree<int> bit(N);
    std::vector<int> p(N);
    if constexpr (is_permutation) {
        std::copy(_begin, _end, p.begin());
    } else {
        std::iota(p.begin(), p.end(), 0);
        std::sort(p.begin(), p.end(), [&](int l, int r) {
            auto litr = std::next(_begin, l), ritr = std::next(_begin, r);
            return *litr != *ritr ? *litr < *ritr : l < r;
        });
    }
    long long ans = 0;
    for (auto x : p) {
        ans += bit.sum(x, N);
        bit.add(x, 1);
    }
    return ans;
}
// void operator>>(istream& is,mint& v){long long x;is>>x;v=x;}
ll N,M,K,H,W,A,B,C,D;
string s,t;
ll ans;
int main(){
  startupcpp();
// int codeforces;cin>>codeforces;while(codeforces--){
  ll i,j;
  cin>>N>>K;
  vec p(N);
  rep(i,N)cin>>p[i];
  auto ch=calcurate_inversion_number(p.begin(),p.end());
  debug(ch);
  ll res=ch/K*K;
  if(res<ch)res+=K;
  debug(res);
  if(res%2!=ch%2)res+=K;
  if(res%2!=ch%2)print(-1);
  else print(res);
// }
}