#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
using namespace std;
#define len(n) (int)n.length()
#define pb push_back
#define ll long long
#define make_pair mkp
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n-1;i >= 0;i--)
#define ALL(v) (v).begin(), (v).end()
#ifdef DBGPRT
#define dbg(n) cerr << "//" << #n << ":" << n <<endl
#else
#define dbg(n) ;
#endif
typedef pair<ll, ll> LLARR;
#define INF 999999999
const double PI = 3.1415926535897932384626433832795;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main()
{
	ios::sync_with_stdio(false);

	ll n,k;
	cin >> n >> k;
	vector<ll> v;
	REP(i,n)
	{
		ll t;cin >> t;
		v.pb(t);
	}
	ll ans = 0;
	REP(i, n-1)
	{
		dbg(i);
		for(int j = i; j < n; j+=k)
		{
			dbg(j);
			if (v[i] > v[j])
			{
				ll tmp = v[i];
				v[i] = v[j];
				v[j] = tmp;
				ans++;
			}
		}
	}
	dbg("");
	for(auto i:v) dbg(i);
	bool f = true;
	REP(i, n-1)
	{
		if (v[i] > v[i+1])
			f = false;
	}
	cout << (f ? ans : -1) << endl;
	return 0;
}