#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#define  _SCL_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream> 
#include<map>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cstdint>
#define REP(i,n) for(int i = 0;i < n;++i)
using namespace std;
int a[2000] = {};
int main() {
	int n, k;
	scanf("%d%d", &n, &k);
	REP(i, n) scanf("%d", &a[i]);
	vector<int> hoge;
	REP(i, n) hoge.push_back(a[i]);
	sort(begin(hoge), end(hoge));
	int ans = 0;
	for (int i = 0;i < n;++i) {
		int where_isa = -1;
		for (int q = 0;q < n;++q) {
			if (hoge[i] == a[q]) {
				where_isa = q;
				goto next;
			}
		}
	next:;
		if (where_isa != i) {
			if (where_isa > i) {
				while (true) {
					ans++;
					where_isa -= k;
					if (where_isa < 0 || where_isa < i) {
						goto cant;
					}
					swap(a[where_isa], a[where_isa + k]);
					if (where_isa == i) break;
				}
			}else{
				while (true) {
					ans++;
					where_isa += k;
					if (where_isa >= n) {
						goto cant;
					}
					swap(a[where_isa], a[where_isa - k]);
					if (where_isa == i) break;
				}
			}
		}
	}
	cout << ans << endl;
	return 0;
cant:;
	cout << "-1" << endl;
	return 0;
}