#include "bits/stdc++.h"
#define _CRT_SECURE_NO_WARNINGS
#define rep(i,n) for(int i = 0;i < n;i++)
#define REP(i,n,k) for(int i = n;i < k;i++)
#define P(p) cout << (p) << endl;
#define sP(p) cout << setprecision(15) << fixed << p << endl;
#define Pi pair<int,int>
#define IINF 1e9
#define LINF 1e18
#define vi vector<int>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
int dx[] = { 1, 0,-1,0 };
int dy[] = { 0, 1,0,-1 };

void bubbleSort(vector<int> v,int k) {
	int count = 0;
	for (int i = 0; i < v.size(); i++) {
		for (int j = 0; j+k < v.size(); j++) {
			if (v[j] > v[j + k]) {
				swap(v[j], v[j + k]);
				count++;
			}
		}
	}
	rep(i, v.size() - 1) {
		if (v[i] > v[i + 1]) {
			cout << -1 << endl;
			return;
		}
	}
	P(count);
}

void solve() {
	int n, k;
	cin >> n >> k;
	vi v;
	rep(i, n) {
		int a;
		cin >> a;
		v.push_back(a);
	}
	bubbleSort(v, k);
}

int main() {
	solve();
	return 0;
}