#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
template <class C>
void mini(C &a4, C b4) {
  a4 = min(a4, b4);
}
template <class C>
void maxi(C &a4, C b4) {
  a4 = max(a4, b4);
}
template <class TH>
void _dbg(const char *sdbg, TH h) {
  cerr << sdbg << '=' << h << endl;
}
template <class TH, class... TA>
void _dbg(const char *sdbg, TH h, TA... a) {
  while (*sdbg != ',') cerr << *sdbg++;
  cerr << '=' << h << ',';
  _dbg(sdbg + 1, a...);
}
template <class T>
ostream &operator<<(ostream &os, vector<T> V) {
  os << "[";
  for (auto vv : V) os << vv << ",";
  return os << "]";
}
template <class L, class R>
ostream &operator<<(ostream &os, pair<L, R> P) {
  return os << "(" << P.first << "," << P.second << ")";
}
struct Sol {
  long long a, b, c, d;
  void run() {
    cin >> a >> b >> c >> d;
    if (a > b * c) {
      cout << "-1\n";
      return;
    }
    long long naj = a / (b * d);
    cout << a * (naj + 1) - (b * d) * naj * (naj + 1) / 2 << "\n";
  }
};
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(11);
  if (0) cout << fixed << setprecision(6);
  long long t;
  cin >> t;
  while (t--) {
    Sol sol;
    sol.run();
  }
}
