#include<iostream>
#include<vector>
#include<algorithm>

int main() {
	int n, k;
	std::cin >> n >> k;

	std::vector<int> v(n);
	for (int i = 0; i < n; i++) {
		std::cin >> v[i];
	}

	std::vector<int> sorted_v = v;
	std::sort(sorted_v.begin(), sorted_v.end());

	long long ans = 0;
	for (int i = 0; i < v.size(); i++) {
		auto d = std::find(v.begin(), v.end(), sorted_v[i]) - v.begin();
		if (d % k == i % k) {
			while (d != i) {
				std::swap(v[d - k], v[d]);
				d -= k;
				ans++;
			}
		} else {
			std::cout << -1 << std::endl;
			return 0;
		}
	}

	std::cout << ans << std::endl;

	return 0;
}