#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;

#define EACH(i,a) for (auto& i : a)
#define FOR(i,a,b) for (ll i=(a);i<(b);i++)
#define RFOR(i,a,b) for (ll i=(b)-1;i>=(a);i--)
#define REP(i,n) for (ll i=0;i<(n);i++)
#define RREP(i,n) for (ll i=(n)-1;i>=0;i--)
#define debug(x) cout<<#x<<": "<<x<<endl
#define pb push_back
#define ALL(a) (a).begin(),(a).end()

const ll linf = 1e18;
const int inf = 1e9;
const double eps = 1e-12;
const double pi = acos(-1);

template<typename T>
istream& operator>>(istream& is, vector<T>& vec) {
	EACH(x,vec) is >> x;
	return is;
}
/*
template<class... T>
ostream& operator<<(ostream& os, tuple<T...>& t) {
	for (size_t i = 0; i < tuple_size< tuple<T...> >::value; ++i) {
		if (i) os << " ";
		os << get<0>(t);
	}
	return os;
}
*/
template<typename T>
ostream& operator<<(ostream& os, vector<T>& vec) {
	REP(i,vec.size()) {
		if (i) os << " ";
		os << vec[i];
	}
	return os;
}
template<typename T>
ostream& operator<<(ostream& os, vector< vector<T> >& vec) {
	REP(i,vec.size()) {
		if (i) os << endl;
		os << vec[i];
	}
	return os;
}

int solve(vector<int> a, int K) {
	int N = a.size();
	vector<int> v = a; sort(ALL(v));
	map<int, int> m;
	REP(i, v.size()) {
		m[a[i]] = i;
	}
	int res = 0;
	REP(i, N) {
		int s = m[v[i]];
		int t = i;
		if ( (s - t) % K != 0 ) return -1;
		for (int j = s; j-K >= t; j -= K) {
			swap(a[j], a[j-K]);
			swap(m[a[j]], m[a[j-K]]);
			++res;
		}
	}
	return res;
}

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(0);
	int N, K; cin >> N >> K;
	vector<int> a(N); cin >> a;
	cout << solve(a, K) << endl;
}