#include<bits/stdc++.h>
using namespace std;
const int MX = (1<<20);
int T , n , arr[MX] , D;
vector < int > v;
bool ok;
long long solve(){
    long long sum = 0;
    vector < pair < int , int > > v1 , v2;
    for(int j = 0 ; j < v.size() ; j++)
        sum += v[j];
    if((sum % v.size())) {
        ok = 0;
        return 0;
    }
    sum /= (v.size());
    for(int j = 0 ; j < v.size() ; j++){
        if(v[j] < sum) v1.push_back({sum - v[j] , j});
        else v2.push_back({v[j] - sum , j});
    }
    reverse(v1.begin() , v1.end());
    reverse(v2.begin() , v2.end());
    long long ret = 0;
    while(!v1.empty()){
        int x = min(v1.back().first , v2.back().first);
        ret += 1ll * x * abs(v2.back().second - v1.back().second);
        v1.back().first -= x;
        v2.back().first -= x;
        if(v1.back().first == 0) v1.pop_back();
        if(v2.back().first == 0) v2.pop_back();
    }
    return ret;

}
int main(){
    scanf("%d",&T);
    while(T--){
        scanf("%d %d",&n,&D);
        ok = 1;
        long long ans = 0;
        for(int j = 0 ; j < n ; j++) scanf("%d",&arr[j]);
        for(int j = 0 ; j < D ; j++){
            v.clear();
            for(int i = j ; i < n ; i+=D){
                v.push_back(arr[i]);
            }
            ans += solve();
        }
        if(!ok) puts("-1");
        else cout<<ans<<endl;

    }

}