#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;

int main()
{
	ll N, K;
	cin >> N >> K;
	ll a[1010];

	for (int i = 0; i < N; i++) {
		cin >> a[i];
	}

	int cnt = 0;
	for (int j = 0; j < 500000; j++) {
		for (int i = 0; i < N-K; i++) {
			if (a[i] > a[i + K]) {
				swap(a[i], a[i + K]);
				cnt++;
			}
		}
	}
	for (int i = 0; i < N-1; i++) {
		if (a[i] > a[i+1])
			cnt = -1;
	}
	cout << cnt << endl;
	return 0;
}