#include "bits/stdc++.h"
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000


int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int N, K;
	cin >> N >> K;
	int a[1010];
	rep(i, N) cin >> a[i];
	int ans = 0;
	rep(i, N - 1) {
		REP(j, 0, N - i - 1 - K) {
			if (a[j] > a[j + K]) {
				ans++;
				swap(a[j], a[j + K]);
			}
			//rep(i, N) cout << a[i] << " ";
			//cout << endl;
		}
	}
	rep(i, N - 1) {
		if (a[i] > a[i + 1]) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << ans << endl;

    return 0;
}