TG I 2

#include <iostream>
#include <cmath>
#include <vector>
#include <queue>
#include <algorithm>
#include <string>
#include <stack>
#include <set>
#include <map>
#define pii pair <int, int>
#define pb(x) push_back(x)
using namespace std;
using ll = long long;
using ld = long double;
using db = double;
void cv(vector <int> &v) {
    for (auto x : v) cout << x << ' ';
    cout << "\n";
}

void cvl(vector <ll> &v) {
    for (auto x : v) cout << x << ' ';
    cout << "\n";
}


void cvv(vector <vector <int> > &v) {
    for (auto x : v) cv(x);
    cout << "\n";
}

void cvb(vector <bool> v) {
    for (bool x : v) cout << x << ' ';
    cout << "\n";
}

void cvs(vector <string>  v) {
    for (auto a : v) {
        cout << a << "\n";
    }
}

void cvp(vector <pii> a) {
    for (auto p : a) {
        cout << p.first << ' ' << p.second << "\n";
    }
    cout << "\n";
}

ll a, b, c, d;
bool sh = 1;

ll S(ll a1, ll d, ll n) {
    return (2 * a1 + d * (n - 1)) * n / 2;
}

ll f(ll t) {
    return 1 + (t - 1)  / d;
}

ll g(ll t) {
    ll k = (t - 1 - c) / d;
    ll a1 = 1 + k * d + c - t;
    ll N = f(t) - (k - 1);
    ll del = S(a1, d * b, N);
    ll res = f(t) * b * c - del;
    return res;
}

ll hp(ll t) {
    return g(t) - f(t) * a ;
}



void slv() {
    cin >> a >> b >> c >> d;
    if (a > b * c) {
        cout << -1 << "\n";
        return;
    }
    //tehnarnik po t
    ll ls = 0, rs = 1e5, s1, s2, mn;
    if (sh) {
        rs = 30;
    }
    while (ls + 3 < rs) {

        s1 = ls + (rs - ls) / 3;
        s2 = ls + 2 * (rs - ls) / 3;
        if (sh) {
            cout << "ls s1 s2 rs = " << ls << ' ' << s1 << ' ' << s2 << ' ' << rs << "\n";
            cout << "hp(s1) = " << hp(s1) << "\n";
            cout << "hp(s2) = " << hp(s2) << "\n";
        }
        if (hp(s1) < hp(s2)) {
            mn = s1;
            rs = s2;
        } else {
            mn = s2;
            ls = s1;
        }
    }
    if (sh) {
        cout << "mn hp(mn) = " << mn << ' ' << hp(mn) << "\n";
    }
    cout << -hp(mn) << "\n";
}



int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int t = 1;
    if (!sh) cin >> t;
    for (int go = 0; go < t; ++go) {
        slv();
    }
}