#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>

using namespace std;

int main() {
	int n, k;

	cin >> n >> k;
	vector<int> a(n, 0), correct;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	correct = a;
	sort(correct.begin(), correct.end());

	int ans = 0;
	for (int i = 0; i < n; i++) {
		int target = correct[i];
		int pos = find(a.begin(), a.end(), correct[i]) - a.begin();

		if ((pos - i) % k != 0) {
			cout << -1 << endl;
			return 0;
		}
		while (pos != i) {
			swap(a[pos], a[pos -k]);
			pos -= k;
			ans++;
		}
	}

	cout << ans << endl;
	return 0;
}