#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) for(int i = (n) - 1; (i) >= 0; --i)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD LL(1e9 + 7)
#define INF 1000000
using namespace std;
typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PI;

int main() {
	int n, k;
	cin >> n >> k;
	VI a(n);
	REP(i, n) 
		cin >> a[i];
	int ans = 0;
	REP(i, n) {
		RREP(j, i) {
			if (n <= j + k)
				break;
			if (a[j] > a[j + k]) {
				swap(a[j], a[j + k]);
				ans++;
			}
		}
	}
	bool bo = true;
	REP(i, n - 1) {
		if (a[i] > a[i + 1])
			bo = false;
	}
	if (bo)
		cout << ans << endl;
	else
		cout << -1 << endl;
	return 0;
}