#include <bits/stdc++.h>
#include <atcoder/all>
#define OVERLOAD_REP(_1, _2, _3, name, ...) name
#define REP1(i,n) for (auto i = std::decay_t<decltype(n)>{}; (i) < (n); ++(i))
#define REP2(i,l,r) for (auto i = (l); (i) != (r); ++(i))
#define rep(...) OVERLOAD_REP(__VA_ARGS__, REP2, REP1)(__VA_ARGS__)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
using namespace std;
using namespace atcoder;
using ll = long long;
using ull = unsigned long long;
using P = pair<int,int>;
using PP = pair<long long,int>;
using LP = pair<long long,long long>;
using mint1 = modint1000000007;
using mint9 = modint998244353;
const int INF = 1'000'000'100;
const long long LINF = 1'000'000'000'000'100'000;
const int di[] = {0,1,0,-1,-1,1,-1,1};
const int dj[] = {1,0,-1,0,1,1,-1,-1};
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;}
void YesNo (bool ok,const string& s="Yes",const string& t="No") {if(ok)cout<<s<<endl;else cout<<t<<endl;}


//-----def end-----//

template<typename T=int>
 struct CC {
   bool initialized;
   vector<T> xs;
   CC(): initialized(false) {}
   void add(T x) { xs.push_back(x);}
   void init() {
     sort(xs.begin(), xs.end());
     xs.erase(unique(xs.begin(),xs.end()),xs.end());
     initialized = true;
   }
   int operator()(T x) {
     if (!initialized) init();
     return upper_bound(xs.begin(), xs.end(), x) - xs.begin() - 1;
   }
   T operator[](int i) {
     if (!initialized) init();
     return xs[i];
   }
   int size() {
     if (!initialized) init();
     return xs.size();
   }
 };

long long f (vector<ll> x) {
     int n = x.size();
     CC cc;
     rep(i , n) cc.add(x[i]);
     int m = cc.size();
     fenwick_tree<ll> d(m);
     long long res = 0;
     rep(i , n) {
         int a = cc(x[i]);
         res += i-d.sum(0 , a+1);
         d.add(a , 1);
     }
     return res;
 }

int main () {
    ll n , m;
    cin >> n >> m;
    vector<ll> p(n);
    rep(i , n) cin >> p[i];
    {
        bool ok = true;
        rep(i , n) if(p[i] != i+1) ok = false;
        if(ok) {
            cout << 0 << endl;
            return 0;
        }
    }
    ll t = f(p) , res;
    if(t%m==0) res = m;
    else res = m*(t/m+1);
    if(res%2 == t%2) cout << res << endl;
    else if ((res+m)%2 == t%2) cout << res+m << endl;
    else cout << -1 << endl;
	return 0;
}