#include <bits/stdc++.h>
using namespace std;

int main(){
    freopen("mosalah.in", "r", stdin);
    long long a, N , M , X , Y;
    cin >> a;
    for (int i = 0; i < a; i++) {
        cin >> N >> M >> X >> Y;
        if (Y == 0 && M < N * (X+Y))
            cout << -1 << endl;
        else if (N * (X+Y) < M)
            cout << 0 << endl;
        else
            cout << N * (X+Y) - M << endl;
    }

    return 0;
}