#include <bits/stdc++.h>
using namespace std;
using i64 = int64_t;
constexpr i64 mod = 998244353;
i64 power(i64 a, i64 r) {
    i64 res = 1;
    for (; r; r >>= 1, a = a * a % mod) {
        if (r & 1) {
            res = res * a % mod;
        }
    }
    return res;
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    i64 n, m;
    cin >> n >> m;
    i64 ans = 0;
    vector<int> bit(n + 1);
    for (int i = 1, p; i <= n; i += 1) {
        cin >> p;
        ans += i - 1;
        for (int x = p; x; x -= x & -x) {
            ans -= bit[x];
        }
        for (int x = p; x <= n; x += x & -x) {
            bit[x] += 1;
        }
    }
    i64 k = (ans + m - 1) / m * m;
    if ((k - ans) % 2 == 0) {
        cout << k;
    } else if ((k + m - ans) % 2 == 0) {
        cout << k + m;
    } else {
        cout << -1;
    }
}