// start A.cpp
// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;

using ll  = long long;
using ull = unsigned long long;
template <class T>
using pq = priority_queue<T>;
template <class T>
using qp = priority_queue<T, vector<T>, greater<T>>;
#define vec(T, A, ...) vector<T> A(__VA_ARGS__);
#define vvec(T, A, h, ...) vector<vector<T>> A(h, vector<T>(__VA_ARGS__));
#define vvvec(T, A, h1, h2, ...) vector<vector<vector<T>>> A(h1, vector<vector<T>>(h2, vector<T>(__VA_ARGS__)));

#ifndef RIN__LOCAL
#define endl "\n"
#endif
#define spa ' '
#define len(A) A.size()
#define all(A) begin(A), end(A)

#define fori1(a) for (ll _ = 0; _ < (a); _++)
#define fori2(i, a) for (ll i = 0; i < (a); i++)
#define fori3(i, a, b) for (ll i = (a); i < (b); i++)
#define fori4(i, a, b, c) for (ll i = (a); ((c) > 0 || i > (b)) && ((c) < 0 || i < (b)); i += (c))
#define overload4(a, b, c, d, e, ...) e
#define fori(...) overload4(__VA_ARGS__, fori4, fori3, fori2, fori1)(__VA_ARGS__)

vector<char> stoc(string &S) {
    int n = S.size();
    vector<char> ret(n);
    for (int i = 0; i < n; i++) ret[i] = S[i];
    return ret;
}

#define INT(...)                                                                                                                                                                                       \
    int __VA_ARGS__;                                                                                                                                                                                   \
    inp(__VA_ARGS__);
#define LL(...)                                                                                                                                                                                        \
    ll __VA_ARGS__;                                                                                                                                                                                    \
    inp(__VA_ARGS__);
#define STRING(...)                                                                                                                                                                                    \
    string __VA_ARGS__;                                                                                                                                                                                \
    inp(__VA_ARGS__);
#define CHAR(...)                                                                                                                                                                                      \
    char __VA_ARGS__;                                                                                                                                                                                  \
    inp(__VA_ARGS__);
#define VEC(T, A, n)                                                                                                                                                                                   \
    vector<T> A(n);                                                                                                                                                                                    \
    inp(A);
#define VVEC(T, A, n, m)                                                                                                                                                                               \
    vector<vector<T>> A(n, vector<T>(m));                                                                                                                                                              \
    inp(A);

const ll MOD1 = 1000000007;
const ll MOD9 = 998244353;

template <class T>
auto min(const T &a) {
    return *min_element(all(a));
}
template <class T>
auto max(const T &a) {
    return *max_element(all(a));
}
template <class T, class S>
auto clamp(T &a, const S &l, const S &r) {
    return (a > r ? r : a < l ? l : a);
}
template <class T, class S>
inline bool chmax(T &a, const S &b) {
    return (a < b ? a = b, 1 : 0);
}
template <class T, class S>
inline bool chmin(T &a, const S &b) {
    return (a > b ? a = b, 1 : 0);
}
template <class T, class S>
inline bool chclamp(T &a, const S &l, const S &r) {
    auto b = clamp(a, l, r);
    return (a != b ? a = b, 1 : 0);
}

void FLUSH() {
    cout << flush;
}
void print() {
    cout << endl;
}
template <class Head, class... Tail>
void print(Head &&head, Tail &&...tail) {
    cout << head;
    if (sizeof...(Tail)) cout << spa;
    print(forward<Tail>(tail)...);
}
template <typename T>
void print(vector<T> &A) {
    int n = A.size();
    for (int i = 0; i < n; i++) {
        cout << A[i];
        if (i != n - 1) cout << ' ';
    }
    cout << endl;
}
template <typename T>
void print(vector<vector<T>> &A) {
    for (auto &row : A) print(row);
}
template <typename T, typename S>
void print(pair<T, S> &A) {
    cout << A.first << spa << A.second << endl;
}
template <typename T, typename S>
void print(vector<pair<T, S>> &A) {
    for (auto &row : A) print(row);
}
template <typename T, typename S>
void prisep(vector<T> &A, S sep) {
    int n = A.size();
    for (int i = 0; i < n; i++) {
        cout << A[i];
        if (i != n - 1) cout << sep;
    }
    cout << endl;
}
template <typename T, typename S>
void priend(T A, S end) {
    cout << A << end;
}
template <typename T>
void priend(T A) {
    priend(A, spa);
}
template <typename T, typename S>
bool printif(bool f, T A, S B) {
    if (f)
        print(A);
    else
        print(B);
    return f;
}
template <class... T>
void inp(T &...a) {
    (cin >> ... >> a);
}
template <typename T>
void inp(vector<T> &A) {
    for (auto &a : A) cin >> a;
}
template <typename T>
void inp(vector<vector<T>> &A) {
    for (auto &row : A) inp(row);
}
template <typename T, typename S>
void inp(pair<T, S> &A) {
    inp(A.first, A.second);
}
template <typename T, typename S>
void inp(vector<pair<T, S>> &A) {
    for (auto &row : A) inp(row.first, row.second);
}

template <typename T>
T sum(vector<T> &A) {
    T tot = 0;
    for (auto a : A) tot += a;
    return tot;
}

template <typename T>
vector<T> compression(vector<T> X) {
    sort(all(X));
    X.erase(unique(all(X)), X.end());
    return X;
}

vector<vector<int>> read_edges(int n, int m, bool direct = false, int indexed = 1) {
    vector<vector<int>> edges(n, vector<int>());
    for (int i = 0; i < m; i++) {
        INT(u, v);
        u -= indexed;
        v -= indexed;
        edges[u].push_back(v);
        if (!direct) edges[v].push_back(u);
    }
    return edges;
}
vector<vector<int>> read_tree(int n, int indexed = 1) {
    return read_edges(n, n - 1, false, indexed);
}
template <typename T>
vector<vector<pair<int, T>>> read_wedges(int n, int m, bool direct = false, int indexed = 1) {
    vector<vector<pair<int, T>>> edges(n, vector<pair<int, T>>());
    for (int i = 0; i < m; i++) {
        INT(u, v);
        T w;
        inp(w);
        u -= indexed;
        v -= indexed;
        edges[u].push_back({v, w});
        if (!direct) edges[v].push_back({u, w});
    }
    return edges;
}
template <typename T>
vector<vector<pair<int, T>>> read_wtree(int n, int indexed = 1) {
    return read_wedges<T>(n, n - 1, false, indexed);
}

inline bool yes(bool f = true) {
    cout << (f ? "yes" : "no") << endl;
    return f;
}
inline bool Yes(bool f = true) {
    cout << (f ? "Yes" : "No") << endl;
    return f;
}
inline bool YES(bool f = true) {
    cout << (f ? "YES" : "NO") << endl;
    return f;
}

inline bool no(bool f = true) {
    cout << (!f ? "yes" : "no") << endl;
    return f;
}
inline bool No(bool f = true) {
    cout << (!f ? "Yes" : "No") << endl;
    return f;
}
inline bool NO(bool f = true) {
    cout << (!f ? "YES" : "NO") << endl;
    return f;
}

// start data_structure/BIT.hpp

template <typename T>
struct BIT {
    int n;
    vector<T> tree;

    BIT(int n) : n(n) {
        tree.assign(n + 1, T(0));
    }
    BIT() {}
    T _sum(int i) {
        i++;
        T res = T(0);
        while (i > 0) {
            res += tree[i];
            i -= i & -i;
        }
        return res;
    }

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

    T sum(int r) {
        return _sum(r - 1);
    }

    T get(int i) {
        return _sum(i) - _sum(i - 1);
    }

    void add(int i, T x) {
        i++;
        while (i <= n) {
            tree[i] += x;
            i += i & -i;
        }
    }

    int lower_bound(T x) {
        int pos  = 0;
        int plus = 1;
        while (plus * 2 <= n) plus *= 2;
        while (plus > 0) {
            if ((pos + plus <= n) && (tree[pos + plus] < x)) {
                x -= tree[pos + plus];
                pos += plus;
            }
            plus >>= 1;
        }
        return pos;
    }
};

// end data_structure/BIT.hpp
// restart A.cpp

void solve() {
    LL(n, m);
    VEC(int, P, n);
    BIT<ll> bit(n);
    ll inv = 0;
    for (auto p : P) {
        p--;
        inv += bit.sum(p, n);
        bit.add(p, 1);
    }
    ll c = (inv + m - 1) / m * m;
    if (c % 2 != inv % 2) c += m;
    if (c % 2 == inv % 2) {
        print(c);
    } else {
        print(-1);
    }
}

int main() {
    cin.tie(0)->sync_with_stdio(0);
    // cout << fixed << setprecision(12);
    int t;
    t = 1;
    // cin >> t;
    while (t--) solve();
    return 0;
}

// end A.cpp