func_code_string: #include <bits/stdc++.h>   using namespace std;   long long a, b, c, d;   long long f(long long k) { return (d * b * k - 2 * a) * (k + 1) / -2; }   int main() {     int t;     cin >> t;     while (t--) {       cin >> a >> b >> c >> d;       if (a > b * c) {         cout <<  -1 n ;         continue;       }       double k = a * 1.0 / (d * b) - 0.5;       cout << max(f(k), f(k + 1)) <<   n ;     }   }   