#include <bits/stdc++.h>

using namespace std;
using ll = long long;

template<class T> struct BIT {
    vector<T> bit;
    int N;

    BIT (int n){
        N = n;
        bit.resize(N);
    }

    void add(int loc, T val){
        loc++;
        while(loc <= N){
            bit[loc-1] += val;
            loc += loc & -loc;
        }
    }

    T sum(int l, int r){
        T res = _sum(r) - _sum(l-1);
        return res;
    }

    T _sum(int r){
        r++;
        T res = 0;
        while(r > 0){
            res += bit[r-1];
            r -= r & -r;
        }
        return res;
    }
};

ll inversion_number(vector<ll> &A){
    ll d=0, mx=0;
    for (int i=0; i<A.size(); i++) mx = max(mx, A[i]);
    BIT<ll> tree(mx+1);

    for (int i=0; i<A.size(); i++){
        d += tree.sum(A[i]+1, mx);
        tree.add(A[i], 1);
    }

    return d;
}

int main(){

    ll N, M, I, K;
    cin >> N >> M;
    vector<ll> A(N);
    for (int i=0; i<N; i++) cin >> A[i];

    I = inversion_number(A);
    K = (I+M-1)/M;
    for (ll i=K; i<=K+1; i++){
        if ((M*K-I) % 2 == 0){
            cout << M*K << endl;
            return 0;
        }
    }

    cout << -1 << endl;

    return 0;
}