#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

int f(vector<int>& a) {
	int N = a.size(), cnt = 0;
	for (int i = 0; i < N; i++)
		for (int j = i + 1; j < N; j++)
			if (a[i] > a[j])
				cnt++;
	return cnt;
}

int main() {
	int N, K; cin >> N >> K;
	vector<int> a(N);
	for (int i = 0; i < N; i++)
		cin >> a[i];
	int ans = 0;
	for (int i = 0; i < K; i++) {
		vector<int> b;
		for (int j = i; j < N; j += K)
			b.push_back(a[j]);
		ans += f(b);
		sort(b.begin(), b.end());
		int k = 0;
		for (int j = i; j < N; j += K)
			a[j] = b[k++];
	}
	for (int i = 0; i + 1 < N; i++)
		if (a[i] > a[i + 1]) {
			cout << -1 << endl;
			return 0;
		}
	cout << ans << endl;
}