#include <bits/stdc++.h>
 
#define endl "\n"
typedef long long ll;
using namespace std;
  
void solve() {
    ll n, k;
    cin >> n >> k;
    unordered_map<int, vector<ll>> mp;
    unordered_map<ll, ll> count;
    for (int i = 0; i < n; i++) {
        ll cur; cin >> cur;
        mp[cur % k].push_back(cur); 
        count[cur]++;
    }

    ll ans = 0, odd_count = 0;
    for (auto pair : mp) {
        vector<ll>& nums = pair.second;
        if (nums.size() % 2 != 0) {
            odd_count ++;
            if (odd_count == 2) {
                cout << "-1" << endl;
                return;
            }
        }

        vector<ll> v;
        for (auto num : nums) {
            if (count[num] % 2 != 0) v.push_back(num);
        }
        ll sz = v.size();
        sort(v.begin(), v.end());
        if (sz % 2 == 1) {
            ll mx_d = 0, index = 0;
            for (int i = 0; i < sz - 2; i+=2) {
                ll dif = abs((v[i + 1] - v[i]) - (v[i + 2] - v[i + 1]));
                if (dif > mx_d) {
                    mx_d = dif; index = i;
                }
            }
            if (v[index + 1] - v[index] < v[index + 2] - v[index + 1]) {
                index = index + 2;
            }
            v.erase(v.begin() + index); sz--;
        }

        for (int i = 0; i < sz - 1; i+=2) {
            ans += (v[i + 1] - v[i]) / k;
        }
    }

    cout << ans << endl;
} 
 
int main() {
ios::sync_with_stdio(0);
    cin.tie(0);
 
    int t;
    cin >> t;
    while (t--) {
        solve();
    }
 
    return 0;
}