#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;


struct Unionfind{
    vector<int> parents;
    int m;
    Unionfind(int n):parents(n,-1){
        m = n;
    }

    int find(int x){
        if(parents.at(x) < 0){
            return x;
        }else{
            parents.at(x) = find(parents.at(x));
            return parents.at(x);
        }
    }

    void unite(int x,int y){
        x = find(x);
        y = find(y);

        if(x==y) return;
        if(parents.at(x) > parents.at(x))swap(x,y);
        parents.at(x) += parents.at(y);
        parents.at(y) = x;
    }  

   int size(int x){
       return -parents.at(find(x));
   }

   bool same(int x,int y){
       return find(x) ==find(y);
   }

   vector<int> members(int x){
       int root = find(x);
       vector<int> A;
       for(int i=0;i<m;i++){
           if(find(i) ==root ){
               A.push_back(i);
           }
       }
      return A;
  }
};

int main(){
    int n,k;
    cin >> n >> k;
    vector<int> a(n);
    rep(i,n) cin >> a[i];
    Unionfind uf(n);
    rep(i,n){
        uf.unite(i,i%k);
    }
    vector<P> A(n);
    rep(i,n){
        A[i].first = a[i];
        A[i].second = i;
    }
    sort(A.begin(),A.end());
    bool ok = true;
    rep(i,n){
        if(!uf.same(A[i].second,i)) ok = false;
    }
    if(!ok){
        cout << -1 << endl;
        return 0;
    }

    int cnt = 0;
    vector<vector<int>> b(k,vector<int>());

    rep(i,n){
        b[i%k].push_back(a[i]);
    }

    rep(i,k){
        while(1){
            bool update = false;
            rep(j,b[i].size()-1){
                if(b[i][j] > b[i][j+1]){
                    swap(b[i][j],b[i][j+1]);
                    update = true;
                    cnt ++;
                }
            }
            if(!update) break;
        }
    }
    cout << cnt << endl;
    return 0;
}