#include <cstdio>

#include <iostream>

#include <cmath>

#include <algorithm>

#include <string>

#include <cstring>

using namespace std;

#define _ ios::sync_with_stdio(false),cin.tie(0)

const int MAXN = 5010;

const int INF = 0xfffffff;

typedef long long ll;

int t;

ll x,y,p,q;

int main()

{

    cin>>t;

    while(t--)

    {

        cin>>x>>y>>p>>q;

        ll l=0,r=1e9,ans=-1;

        while(l<=r)

        {

            ll mid=(l+r)>>1;

            ll a=mid*p-x,b=mid*q-y;

            if(a>=0&&b>=0&&a<=b)

            {

                ans=mid;

                r=mid-1;

            }

            else

                l=mid+1;

        }

        if(ans==-1)

            cout<<-1<<endl;

        else

            cout<<ans*q-y<<endl;

    }

    return 0;

}