func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     int T;     cin >> T;     while (T--) {       long long a, b, c, d;       cin >> a >> b >> c >> d;       if (c <= d) {         if (a <= b * c)           cout << a << endl;         else {           cout << -1 << endl;         }       } else {         long long len = c % d ? c / d + 1 : c / d;         long long ans = 0;         long long x = (b * d + 2 * a) / 2 / b / d;         x = min(x, len);         ans = a * x - x * (x - 1) / 2 * b * d;         x++;         x = min(x, len);         ans = max(ans, a * x - x * (x - 1) / 2 * b * d);         if (a > b * c)           cout << -1 << endl;         else {           ans = max(ans, a * len - (len - 1) * len / 2 * d * b);           cout << ans << endl;         }       }     }   }   