#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
#define drep(i, cc, n) for (ll i = (cc); i <= (n); ++i)
#define rep(i, n) drep(i, 0, n - 1)
#define all(a) (a).begin(), (a).end()
#define pb push_back
#define fi first
#define se second

mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
const ll MOD1000000007 = 1000000007;
const ll MOD998244353 = 998244353;
const ll MOD[3] = {999727999, 1070777777, 1000000007};
const ll LINF = 1LL << 60;
const int IINF = 1 << 30 - 1;

template<typename T> struct Edge{
    int to; T w;
    Edge(int to_, T w_=1){
        to = to_;
        w=w_;
    }
};
template<typename T> using Tree = vector<vector<Edge<T>>>;
template<typename T> using Graph = vector<vector<Edge<T>>>;
/* 容量&重み付きエッジ for Dinic */
template<typename T> struct REdge{
    int to;
    T cap;
    T cost;
    int rev;
    REdge(int to_, T cap_, T cost_=1){
        to = to_;
        cap = cap_;
        cost = cost_;
    }
    
    REdge(int to_, T cap_, T cost_, int rev_){
        to = to_;
        cap = cap_;
        cost = cost_;
        rev = rev_;
    }
};

/* 残余グラフ for Dinic */
template<typename T> using RGraph = vector<vector<REdge<T>>>;

struct UnionFind{
    vector<ll> parent;
    vector<ll> sizes;
    
    UnionFind(ll N) : parent(N), sizes(N, 1){
        rep(i, N) parent[i] = i;
    }
    
    ll root(ll x){
        if (parent[x] == x) return x;
        return parent[x] = root(parent[x]);
    }
 
    void unite(ll x, ll y){
        ll rx = root(x);
        ll ry = root(y);
        if (rx == ry) return;
        if (sizes[rx] < sizes[ry]) swap(rx, ry);
        sizes[rx] += sizes[ry];
        parent[ry] = rx;
    }
 
    bool same(ll x, ll y){
        ll rx = root(x);
        ll ry = root(y);
        return rx == ry;
    }
 
    ll size(ll x){
        return sizes[root(x)];
    }
};

struct SegmentTree{
    ll n;
    vector<ll> dat;

    SegmentTree(ll n_){
        n = 1;
        while(n < n_) n*=2;
        dat.resize(2*n, 0);
    }

    void update(ll k, ll a){
        k += n-1;
        dat[k] += a;
        while(k > 0){
            k = (k-1)/2;
            dat[k] = dat[2*k+1]+dat[2*k+2];
        }
    }

    // the minimun element of [a, b)
    ll query(ll a, ll b){return query_sub(a, b, 0, 0, n);}

    ll query_sub(ll a, ll b, ll k, ll l, ll r){
        if(r <= a || b <= l){
            return 0;
        }else if(a <= l && r <= b){
            return dat[k];
        }else{
            ll vl = query_sub(a, b, 2*k+1, l, (l+r)/2);
            ll vr = query_sub(a, b, 2*k+2, (l+r)/2, r);
            return vl+vr;
        }
    }
};

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    ll n, m; cin >> n >> m;
    vector<ll> p(n);
    rep(i, n){
        cin >> p[i];
        p[i]--;
    }

    ll cnt = 0;
    SegmentTree seg(n);
    rep(i, n){
        cnt += seg.query(p[i]+1, n);
        seg.update(p[i], 1);
    }

    if(m%2==0){
        if(cnt%2==1){
            cout << -1 << endl;
        }else{
            cout << cnt + (m-cnt%m)%m << endl;
        }
    }else{
        ll l = cnt + (m-cnt%m)%m;
        if((l-cnt)%2==0){
            cout << l << endl;
        }else{
            cout << l+m << endl;
        }
    }
}