#include <iostream>
#include <vector>
#include <math.h>
#include <string>
#include <deque>
#include <utility>
#include <sstream>
#include <set>
#include <algorithm>
#include <numeric>
#include <list>
#define FIO ios::sync_with_stdio(0);
# define NEW(type, count) ((type *)calloc(count, sizeof(type)))
using namespace std;
struct sett{int n,m,x,y;};
int main(){
  //freopen("mosalah.in" , stdin, "r");
  int n;
  cin>>n;
  vector <sett> z(n);
  for (int i=0;i<n;i++){
    cin>>z[i].n>>z[i].m>>z[i].x>>z[i].y;
  }
  for (int i=0;i<n;i++){
    int ans = ((z[i].x+z[i].y)*z[i].n)-z[i].m,curravg,avg;
    if (z[i].m == 0){
     curravg=0;
    }else{
     curravg = z[i].x/z[i].m;
    }
    if (z[i].n == 0){
      avg=0;
    }else{
     avg = (z[i].x+z[i].y)/z[i].n;
    }
    if (z[i].y == 0){
      cout<< -1<<endl;
    }else if (curravg>avg||curravg==avg||ans<=0){
      cout << 0<<endl;
    }
    else{
      cout<<ans<<endl;
    }
  }

}