/* Problem URL: https://codeforces.com/problemset/problem/1986/E */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template <class T, class comp = less<>>
using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

const int oo = INT32_MAX >> 1;
const ll OO = INT64_MAX >> 1;


void pre()
{

}

#define TEST 1

void solve()
{
    int n;
    ll k;
    cin >> n >> k;
    vl a(n);
    cin >> a;

    map<ll, vl> c;
    rep(i, n) {
        c[a[i] % k].push_back(a[i]);
    }

    int oc = 0;
    repv(i, c) {
        oc += i.second.size() & 1;
    }

    if (oc > 1) {
        cout << "-1\n";
        return;
    }

    auto func = [&](vl &v) -> int {
        if (v.size() <= 1) {
            return 0;
        }
        int n = v.size();
        vl suf(n + 1);
        suf[n - 2] = (v.end()[-1] - v.end()[-2]) / k;
        for (int i = n - 4; i >= 0; i -= 2) {
            suf[i] = suf[i + 2] + (v[i + 1] - v[i]) / k;
        }

        ll ans = OO;
        ll sum = 0;
        rep(i, n) {
            if (i & 1) {
                rmin(ans, (v[i + 1] - v[i - 1]) / k + suf[i + 2] + sum);
                sum += (v[i] - v[i - 1]) / k;
                continue;
            }

            rmin(ans, sum + suf[i + 1]);
        }

        return ans;
    };

    ll ans = 0;
    repv(i, c) {
        auto &v = i.second;
        sortv(v);
        if (v.size() & 1) {
            ans += func(v);
            continue;
        }

        nrep(j, 1, v.size()) {
            ans += (v[j] - v[j - 1]) / k;
            j++;
        }
    }

    cout << ans << '\n';
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    pre();

    int t;
    (TEST && cin >> t) || (t = 1);
    while (t--) {
        solve();
    }
}