#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)

int main()
{
	int n, k; cin >> n >> k;
	vector<int> a(n); rep(i, 0, n) cin >> a[i];
	vector<int> b(n); rep(i, 0, n) b[i] = a[i];
	sort(b.begin(), b.end());

	int ans = 0;
	rep(i, 0, n)
	{
		int j = 0;
		rep(k, 0, n) if (a[k] == b[i]) j = k;

		int d = j - i;
		if (d % k != 0)
		{
			cout << -1 << endl;
			return 0;
		}

		while (i != j)
		{
			swap(a[j], a[j - k]);
			j -= k;
			ans++;
		}
	}
	cout << ans << endl;
}