#include <bits/stdc++.h>
using namespace std;

int main() {
	int n, K;
	cin >> n >> K;

	vector<int> a(n);
	for (int i = 0; i < n; i++) cin >> a[i];

	int ans = 0;
	for (int i = 0; i < K; i++) {
		vector<int> s;
		for (int j = i; j < n; j += K) s.push_back(a[j]);
		for (int j = 0; j < s.size(); j++) {
			for (int k = 0; k < j; k++) {
				if (s[k] > s[j]) ans++;
			}
		}
		sort(s.begin(), s.end());
		for (int j = i, k = 0; j < n; j += K, k++) a[j] = s[k];
	}
	if (!is_sorted(a.begin(), a.end())) ans = -1;
	cout << ans << endl;
}