#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <iomanip>
#include <utility>
#include <tuple>
#include <functional>
#include <bitset>
#include <cassert>
#include <complex>
#include <stdio.h>
#include <time.h>
#include <numeric>
#include <random>
#include <unordered_set>
#include <unordered_map>
#define all(a) (a).begin(), (a).end()
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define req(i, a, b) for (ll i = (a); i < (b); i++)
#define pb push_back
#define debug(x) cerr << __LINE__ << ' ' << #x << ':' << (x) << '\n'
#define debug2(x, y) cerr << __LINE__ << ' ' << #x << ':' << (x) << ',' << #y << ':' << (y) << '\n'
#define debug3(x, y, z) cerr << __LINE__ << ' ' << #x << ':' << (x) << ',' << #y << ':' << (y) << ',' << #z << ':' << (z) << '\n'
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef long double ld;
template<class T> using P = pair<T, T>;
template<class T> using pri_s = priority_queue<T, vector<T>, greater<T>>;
template<class T> using pri_b = priority_queue<T>;
constexpr int inf = 1000000010;
constexpr int inf2 = 2000000010;
constexpr ll INF = 1000000000000000010;
constexpr ll INF4 = 4000000000000000010;
constexpr int mod1e9 = 1000000007;
constexpr int mod998 = 998244353;
constexpr ld eps = 1e-12;
constexpr ld pi = 3.141592653589793238;
constexpr ll ten(int n) { return n ? 10 * ten(n - 1) : 1; };
int dx[] = { 1,0,-1,0,1,1,-1,-1,0 }; int dy[] = { 0,1,0,-1,1,-1,1,-1,0 };
ll mul(ll a, ll b) { return (b != 0 && a > INF / b ? INF : a * b); }
void fail() { cout << "-1\n"; exit(0); } void no() { cout << "No\n"; exit(0); }
template<class T> void er(T a) { cout << a << '\n'; exit(0); }
template<class T, class U> inline bool chmax(T &a, const U &b) { if (a < b) { a = b; return true; } return false; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if (a > b) { a = b; return true; } return false; }
template<class T> istream &operator >>(istream &s, vector<T> &v) { for (auto &e : v) s >> e; return s; }
template<class T> ostream &operator <<(ostream &s, const vector<T> &v) { for (auto &e : v) s << e << ' '; return s; }
template<class T, class U> ostream &operator << (ostream &s, const pair<T, U> &p) { s << p.first << ' ' << p.second; return s; }

struct fastio {
	fastio() {
		cin.tie(0); cout.tie(0);
		ios::sync_with_stdio(false);
		cout << fixed << setprecision(20);
		cerr << fixed << setprecision(20);
	}
}fastio_;

template<class T> class segtree {
	int n;
	vector<T> data;
	T id = 0;
	T operation(T a, T b) { return a + b; }
public:
	segtree(ll _n) {
		n = 1;
		while (n < _n + 2) n <<= 1;
		data = vector<T>(2 * n, id);
	}
	segtree(vector<T> vec) {
		int _n = vec.size();
		n = 1;
		while (n < _n + 2) n <<= 1;
		data = vector<T>(2 * n, id);
		for (int i = 0; i < _n; i++) data[i + n] = vec[i];
		for (int i = n - 1; i >= 1; i--) data[i] = operation(data[i << 1], data[i << 1 | 1]);
	}
	void change(int i, T x) {
		i += n;
		data[i] = x;
		while (i > 1) {
			i >>= 1;
			data[i] = operation(data[i << 1], data[i << 1 | 1]);
		}
	}
	void add(int i, T x) { change(i, data[i + n] + x); }
	T get(int a, int b) {
		T left = id; T right = id;
		a += n; b += n;
		while (a < b) {
			if (a & 1) left = operation(left, data[a++]);
			if (b & 1) right = operation(data[--b], right);
			a >>= 1; b >>= 1;
		}
		return operation(left, right);
	}
	T get_all() { return data[1]; }
	T operator[](int i) { return data[i + n]; }
};

int main() {
	ll n, m;
	cin >> n >> m;
	vector<ll> p(n);
	cin >> p;
	ll cnt = 0;
	segtree<ll> seg(n);
	rep(i, n){
		p[i]--;
		cnt += seg.get(p[i] + 1, n);
		seg.add(p[i], 1);
	}
	if(m % 2 == 0 && cnt % 2 == 1) fail();
	else if(m % 2 == 0){
		ll r = cnt % m;
		if(r) cnt += m - r;
		cout << cnt << '\n';
	}
	else {
		ll r = cnt % m;
		if(r){
			if(r % 2 == 0) cnt += 2 * m - r;
			else cnt += m - r;
		}
		cout << cnt << '\n';
	}
}