#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>

//#include<bits/stdc++.h>



using namespace std;


int main() {
	int n, k;
	cin >> n >> k;
	vector<int>a, b;
	for (int i = 0; i < n; i++) {
		int r;
		cin >> r;
		a.push_back(r), b.push_back(r);
	}
	sort(b.begin(), b.end());
	int s = 0;
	vector<int>x[1030],y[1030];
	for (int i = 0; i < n; i++) {
		x[i%k].push_back(a[i]);
		y[i%k].push_back(b[i]);
	}
	for (int i = 0; i < k; i++) {
		for (int j = 0; j < x[i].size(); j++) {
			for (int w = x[i].size() - 1; w > j; w--) {
				//cout << x[i][w];
				if (x[i][w] < x[i][w - 1]) {
					swap(x[i][w], x[i][w - 1]);
					s++;
				}
			}
		}
	}
	for (int i = 0; i <= 1000; i++) {
		if (x[i] != y[i]) {
			cout << -1 << endl;
			return 0;
		}
	}
	
	cout << s << endl;

	return 0;


}