#include "bits/stdc++.h"

#define REP(i, n) for(decltype(n) i = 0; i < (n); i++)
#define REP2(i, x, n) for(decltype(x) i = (x); i < (n); i++)
#define RREP(i, n) for (decltype(n) i = (n) - 1;i >= 0; i--)

#define ALL(a) (a).begin(),(a).end()
#define SORT(c) sort((c).begin(),(c).end())
#define DESCSORT(c) sort(c.begin(), c.end(), greater<int>())

#define LL long long int
#define LD long double

#define PI 3.14159265358979

#define _CRT_SECURE_NO_WARNINGS

using namespace std;

//================================================

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	int N, K, ans = 0;
	cin >> N >> K;
	vector<int>vec(N);

	REP(i, N) cin >> vec[i];
	vector<int>sortVec = vec;
	SORT(sortVec);

	if (vec == sortVec) {
		cout << "0" << "\n";
		return 0;
	}

	REP(i, N) {
		for (int j = 0; j + K < N; j++) {
			if (vec[j] > vec[j + K]) {
				swap(vec[j], vec[j + K]);
				ans++;
			}
		}
	}

	REP(i, N - 1) {
		if (vec[i] > vec[i + 1]) {
			cout << "-1" << "\n";
			return 0;
		}
	}

	cout << ans << "\n";
	return 0;
}