#include <bits/stdc++.h>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<30;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-8;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};

int n,k;
vvi a;

int main(){
	cin>>n>>k;
	a=vvi(k);
	for(int i=0;i<n;i++){
		int A;
		cin>>A;
		a[i%k].push_back(A);
	}
	int res=0;
	for(int i=0;i<k;i++){
		vi b=a[i];
		int N=b.size();
		while(1){
			bool B=1;
			for(int j=1;j<N;j++) if(b[j-1]>b[j]){
				B=0;res++;
				swap(b[j-1],b[j]);
				break;
			}
			if(B) break;
		}
		a[i]=b;
	}
	for(int i=1;i<n;i++) if(a[(i-1)%k][(i-1)/k]>a[i%k][i/k]) res=-1;
	cout<<res<<endl;
}