#include "bits/stdc++.h"
#define REP(i,n,N) for(ll i=(n); i<(N); i++)
#define RREP(i,n,N) for(ll i=(N-1); i>=n; i--)
#define CK(n,a,b) (a)<=(n)&&(n)<(b)
#define p(s) cout<<(s)<<endl
#define first F
#define second S
typedef long long ll;
using namespace std;
const ll inf =1e9+7;

int N,K;
int a[1010];
int ans;
int main(){
    while(cin>>N>>K){
        REP(i,0,N){
            cin>>a[i];
        }
        REP(j,0,2*N){
            REP(i,0,N-K){
                if(a[i]>a[i+K]){
                    swap(a[i],a[i+K]);
                    ans++;
                }
            }
        }
        REP(i,0,N-1){
            if(a[i]>a[i+1]){
                ans=-1;
                break;
            }
        }
        p(ans);
    }

    return 0;
}