#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
long long op(long long a, long long b) { return a + b; }
long long e() { return 0; }
long long calc(vector<int>v) {
	auto tmp = v;
	sort(all(tmp));
	tmp.erase(unique(all(tmp)), tmp.end());
	rep(i, v.size()) v[i] = lower_bound(all(tmp), v[i]) - tmp.begin();
	long long ret = 0;
	segtree<long long, op, e>seg(tmp.size());
	rep(i, v.size()) {
		ret += seg.prod(v[i] + 1, tmp.size());
		seg.set(v[i], seg.get(v[i]) + 1);
	}
	return ret;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	long long m; cin >> m;
	vector<int>p(n);
	rep(i, n)cin >> p[i], p[i]--;
	long long x = calc(p);
	long long y = ((x + m - 1) / m)*m;
	long long z = y + m;
	if (0 == abs(y - x) % 2) cout << y << endl;
	else if (0 == abs(z - x) % 2) cout << z << endl;
	else cout << -1 << endl;
	return 0;
}