#include <bits/stdc++.h>
using namespace std;
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...);
}
const long long mod = 998244353;
const int inf = 1 << 30;
const int maxn = 1 << 21;
int n, m;
void err() {
  cout << "-1\n";
  exit(0);
}
long long a, b, c, d;
void solve() {
  cin >> a >> b >> c >> d;
  if (a > b * c) {
    cout << -1 << "\n";
    return;
    ;
  }
  long long ti = a / b / d;
  cout << (2 * a - d * b * ti) * (ti + 1) / 2 << "\n";
}
int main() {
  int tc = 1;
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> tc;
  for (int tt = 0; tt < (int)(tc); ++tt) {
    solve();
  }
}
