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

int main()
{
    int t;
    cin >> t;
    while(t--)
    {
        long long int x,y,p,q,g,h,i;
        cin >> x >> y >> p >> q;
        long long int yy,xx,b;
        if(p==0 && x==0) cout << 0 << endl;
        else if(p==0 && x!=0) cout << -1 << endl;
        else if(y-x==0 && p-q==0)cout << 0 << endl;
        else if(y-x!=0 && p-q==0)cout << -1 << endl;
        else
        {
            g=(y-x)/(q-p);
            h=y/q;
            i=x/p;
            b=max({g,h,i});
            //cout << b << y-x << q-p << endl;
            while(1)
            {
                yy=(q*b)-y;
                xx=(p*b)-x;
                if(yy<0 || xx<0)
                {
                    b++;
                    continue;
                }
                if(yy>=xx)
                {
                    cout << yy << endl;
                    break;
                }
                b++;
                //cout << b << endl;
            }
        }
    }
}