#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1ll<<29;
const ll MOD = 1000000007;
const double EPS  = 1e-10;

int main() {
	int n, k;
	int a[1000];
	
	cin >> n >> k;
	REP(i, n) scanf("%d", a + i);
	
	int cnt = 0;
	
	REP(i, n) REP(j, n - i - k) {
		if (a[j] > a[j + k]) {
			swap(a[j], a[j + k]);
			cnt++;
		}
	}
	
	REP(i, n - 1) if (a[i] > a[i + 1]) cnt = -1;
	cout << cnt << endl;
	
	return 0;
}