#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vvi = vector<vector<int>>;
using vi = vector<int>;
using vvll = vector<vector<long long>>;
using vll = vector<long long>;
using vd = vector<double>;
using vvd = vector<vector<double>>;
using pii = pair<int, int>;
using vpii = vector<pair<int, int>>;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
void solve() {
  ll a, b, c, d;
  cin >> a >> b >> c >> d;
  if (a > b * c) {
    cout << -1 << endl;
    return;
  }
  ll diff = d * b;
  ll terms = a / diff + 1;
  ;
  ll ans = ((2 * a + (terms - 1) * -diff) * terms) / 2;
  cout << ans << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int tc;
  cin >> tc;
  while (tc--) {
    solve();
  }
  return 0;
}
