#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <ostream>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define INF 1000000000
#define rep(i,a,b) for (int i=(a);i<(b);i++)
#define rev(i,a,b) for (int i=(a)-1;i>=b;i--)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef queue<int> qi;
typedef vector<int> vi;
typedef vector<string> vs;
int dx[4] = { 0, 1, 0, -1 };
int dy[4] = { 1, 0, -1, 0 };

int n, k;
ll a[1000];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin >> n >> k;
	rep(i,0,n) cin >> a[i];

	ll ans = 0;
	bool flag = true;
	while(flag) {
		flag = false;
		rep(i,0,n/2) {
			if(i+k < n && a[i] > a[i+k]) {
				ll tmp = a[i];
				a[i] = a[i+k];
				a[i+k] = tmp;
				ans++;
				flag = true;
			}
		}
	}

	rep(i,1,n)
		if(a[i-1] > a[i]) {
			cout << "-1" << endl;
			return 0;
		}
	cout << ans << endl;

	return 0;
}