#include <bits/stdc++.h>

using namespace std;

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n, k, t;
	vector <int> a, b;

	cin >> n >> k;

	for (int i = 0; i < n; i++)
	{
		cin >> t;
		a.push_back(t);
	}

	b = a;
	sort(b.begin(), b.end());

	int res = 0;

	for (int i = 0; i < n; i++)
	{
		int p = -1;
		for (int j = i; j < n; j++)
		{
			if (b[i] == a[j])
			{
				p = j;
				break;
			}
		}
		if ((p - i) % k != 0)
		{
			cout << -1 << '\n';
			return 0;
		}
		res += ((p - i) / k);
		for (int j = p; j > i; j -= k)
		{
			int tt = a[j];
			a[j] = a[j - k];
			a[j - k] = tt;
		}
	}
	cout << res << '\n';
	return 0;
}