#include <bits/stdc++.h>
#ifdef LOCAL
#include <debug.hpp>
#else
#define debug(...) void(0)
#endif

#include "atcoder/fenwicktree"

using namespace std;

typedef long long ll;
#define all(x) begin(x), end(x)
constexpr int INF = (1 << 30) - 1;
constexpr long long IINF = (1LL << 60) - 1;
constexpr int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

template <class T> istream& operator>>(istream& is, vector<T>& v) {
    for (auto& x : v) is >> x;
    return is;
}

template <class T> ostream& operator<<(ostream& os, const vector<T>& v) {
    auto sep = "";
    for (const auto& x : v) os << exchange(sep, " ") << x;
    return os;
}

template <class T, class U = T> bool chmin(T& x, U&& y) { return y < x and (x = forward<U>(y), true); }

template <class T, class U = T> bool chmax(T& x, U&& y) { return x < y and (x = forward<U>(y), true); }

template <class T> void mkuni(vector<T>& v) {
    sort(begin(v), end(v));
    v.erase(unique(begin(v), end(v)), end(v));
}

template <class T> int lwb(const vector<T>& v, const T& x) { return lower_bound(begin(v), end(v), x) - begin(v); }

template <class T> T ceil(T x, T y) {
    assert(y >= 1);
    return (x > 0 ? (x + y - 1) / y : x / y);
}
template <class T> T floor(T x, T y) {
    assert(y >= 1);
    return (x > 0 ? x / y : (x - y + 1) / y);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N;
    ll M;
    cin >> N >> M;
    atcoder::fenwick_tree<int> ft(N);
    ll inv = 0;
    for (int i = 0; i < N; i++) {
        int P;
        cin >> P;
        P--;
        inv += ft.sum(P + 1, N);
        ft.add(P, 1);
    }

    ll ans = ceil(inv, M) * M;
    for (int i = 0; i < 2; i++, ans += M) {
        if ((ans - inv) % 2 == 0) {
            cout << ans << '\n';
            return 0;
        }
    }
    cout << -1 << '\n';
    return 0;
}