#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
using namespace std;

#define ll long long
#define INF (1 << 30)
#define INFLL (1LL << 60)

int n,a[1010] = {},ans = 0,b[1010],c;

int main() {
	cin >> n >> c;
	for(int i = 0;i < n;i++){
		cin >> a[i];
		b[i] = a[i];
	}
	sort(b,b + n);
	bool flag = false;
	for(int now = 0;now < n;now++){
		for(int i = 0;i < n;i++){
			if(a[i] == b[now]){
				int p = i;
				while(a[p] != b[p]){
					if(a[p] < b[p]){
						if(p - c < 0 || b[p - c] < a[p]) flag = true;
						swap(a[p],a[p - c]);
						p -= c;
					}else if(a[p] > b[p]){
						if(p + c >= n || b[p + c] > a[p]) flag = true;
						swap(a[p],b[p + c]);
						p += c;
					}
					if(flag) break;
					ans++;
				}
			}
			if(flag) break;
		}
		if(flag) break;
	}
	if(flag) cout << -1 << endl;
	else cout << ans << endl;
	return 0;
}