#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MOD (998244353)

class BIT
{
    int n;
    vector<int> a;

public:
    BIT(int n_) : n(n_)
    {
        a.resize(n + 1, 0);
    }
    void add(int i, int x)
    {
        while (i <= n)
        {
            a[i] += x;
            i += i & (-i);
        }
    }
    int sum(int i)
    {
        int ret = 0;
        while (i > 0)
        {
            ret += a[i];
            i -= i & (-i);
        }
        return ret;
    }
    int sum(int l, int r)
    {
        return sum(r) - sum(l - 1);
    }
};

ll modpow(ll a, ll x)
{
    ll ret = 1;
    a %= MOD;
    while (x > 0)
    {
        if (x % 2 == 1)
        {
            ret = ret * a % MOD;
        }
        a = a * a % MOD;
        x >>= 1;
    }
    return ret;
}

int main()
{
    int N;
    ll M;
    cin >> N >> M;
    vector<int> P(N);
    BIT tree(N);
    ll sum = 0;
    for (int i = 0; i < N; i++)
    {
        cin >> P[i];
        sum += (ll)tree.sum(P[i] + 1, N);
        tree.add(P[i], 1);
    }

    if (sum == 0)
    {
        cout << 0 << endl;
        return 0;
    }

    if (sum % 2 == 0)
    {
        ll ans = ((sum - 1) / M + 1) * M;
        if (ans % 2 == 1)
        {
            ans += M;
        }
        cout << ans << endl;
    }
    else
    {
        if (M % 2 == 0)
        {
            cout << -1 << endl;
        }
        else
        {
            ll ans = ((sum - 1) / M + 1) * M;
            if (ans % 2 == 0)
            {
                ans += M;
            }
            cout << ans << endl;
        }
    }
}