#include <bits/stdc++.h>
using namespace std;
using ll = long long;

template <class T> struct fenwick_tree {
    using U = T;

    public:
    fenwick_tree() : _n(0) {}
    fenwick_tree(int n) : _n(n), data(n) {}

    void add(int p, T x) {
        assert(0 <= p && p < _n);
        p++;
        while (p <= _n) {
            data[p - 1] += U(x);
            p += p & -p;
        }
    }

    T sum(int l, int r) {
        assert(0 <= l && l <= r && r <= _n);
        return sum(r) - sum(l);
    }

    private:
    int _n;
    std::vector<U> data;

    U sum(int r) {
        U s = 0;
        while (r > 0) {
            s += data[r - 1];
            r -= r & -r;
        }
        return s;
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll m, cnt = 0, cnt2;
    int n, v;
    cin >> n >> m;
    fenwick_tree<int> fw(n);
    for(int i = 0; i < n; i++){
        cin >> v;
        cnt += fw.sum(v--, n);
        fw.add(v, 1);
    }
    cnt2 = (cnt + m - 1) / m * m;
    if((cnt2 - cnt) % 2 == 0){
        cout << cnt2 << '\n';
        return 0;
    }
    cnt2 += m;
    cout << (cnt2 ^ cnt & 1 ? -1 : cnt2) << '\n';
}