#define _GLIBCXX_FILESYSTEM
#include <bits/stdc++.h>
#define IOS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ll long long
#define f first
#define s second
 
using namespace std;
 
 
int n;
 
int main()
{
 
 
    IOS
    int t;
    cin>>t;
    while(t--)
    {
        int k;
        cin>>n>>k;
        vector<int> vec(n+1);
        for(int i=1; i<=n; i++)
            cin>>vec[i];
        if(n==1)
        {
            cout<<0<<"\n";
            continue;
        }
 
        sort(vec.begin()+1,vec.end());
 
        ll out=LLONG_MAX;
        for(int j=1; j<=n; j++)
        {
            map<int,int> mp;
            int mtch=0;
            ll ans=0;
            for(int i=1; i<=n; i++)
            {
                if(i==j&&n&1)continue;
                ll md=vec[i]%k;
                if(mp[md]==0)
                {
                    mp[md]=i;
                    mtch++;
                }
                else
                {
                    ll clc=abs(vec[mp[md]]-vec[i])/k;
                    ans+=clc;
                    mp[md]=0;
                    mtch--;
                }
            }
            if(mtch==0)
                out=min(out,ans);
 
            if(n&1==0) break;
 
        }
 
        if(out==LLONG_MAX)cout<<"-1\n";
        else cout<<out<<"\n";
 
 
 
    }
 
 
}
 
 
 

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11
1 1000000000
1
2 1
624323799 708290323
3 1
3 2 1
4 1
7 1 5 3
5 1
11 2 15 7 10
7 1
1 8 2 16 8 16 31
13 1
2 1 1 3 3 11 12 22 45 777 777 1500 74
10 2
1 2 1 2 1 2 1 2 1 2
11 2
1 2 1 2 1 2 1 2 1 2 1
13 3
2 3 9 14 17 10 22 20 18 30 1 4 28
5 1
2 3 5 3 5