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

#define int long long
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define each(it,v) for(__typeof((v).begin()) it=(v).begin();it!=(v).end();it++)
template<class T,class U>inline void chmin(T &t,U f){if(t>f)t=f;}
template<class T,class U>inline void chmax(T &t,U f){if(t<f)t=f;}

int N,K;
vint A[1000];
signed main(){
    cin>>N>>K;
    rep(i,N){
        int a;
        cin>>a;
        A[i%K].pb(a);
    }

    int cnt=0;
    rep(k,K){
        rep(i,A[k].size()){
            reps(j,i+1,A[k].size()){
                if(A[k][i]>A[k][j]){
                    cnt++;
                    swap(A[k][i],A[k][j]);
                }
            }
        }
    }
    vint v;
    rep(i,N){
        v.pb(A[i%K][i/K]);
    }

    rep(i,N-1)if(v[i]>v[i+1]){
    cout<<-1<<endl;
    return 0;}
    cout<<cnt<<endl;
    return 0;
}