#include <bits/stdc++.h>
using namespace std;

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef pair<LL, LL> PLL;

#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second
template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}

const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

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

  int N, K; cin >> N >> K;
  VI xs(N); REP(i,N) cin >> xs[i];

  int ans = 0;
  for(int i=0;i<K;++i){
	bool up = true;
	while(up){
	  up = false;
	  for(int j=i+K;j<N;j+=K){
		if(xs[j-K] > xs[j]){
		  swap(xs[j-K], xs[j]);
		  ++ans;
		  up = true;
		}
	  }
	}
  }

  bool sorted = true;
  REP(i,N-1) if(xs[i] > xs[i+1]) sorted = false;
  cout << (sorted? ans: -1) << endl;
  
  return 0;
}