【題目敘述】http://codeforces.com/contest/1413/problem/E
#include <bits/stdc++.h>
using namespace std;
long long t, a, b, c, d;
int main(){
cin >> t;
while (t--){
cin >> a >> b >> c >> d;
if (a > b*c){
cout << -1 << "\n";
continue;
}
long long i = a/(d*b)+1;
cout << i*a-((i*i*d-i*d)/2)*b << "\n";
}
}