#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long a, b, c, d;
  cin >> a >> b >> c >> d;
  if (a > b * c) {
    cout << "-1\n";
    return;
  }
  if (c < d) {
    cout << a << "\n";
    return;
  }
  long long mx = c / d;
  long long k = (2 * a - b * d) / (2 * d * b);
  long long ans = 0;
  for (long long i = k - 1; i <= k + 1; i++) {
    if (i < 0 || i > mx) continue;
    ans = max(ans, (i + 1) * a - i * (i + 1) / 2 * b * d);
  }
  cout << ans << "\n";
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout << fixed;
  cout.precision(30);
  long long t = 1;
  cin >> t;
  while (t--) solve();
}
