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

vector<long long> calc_divisors(long long N) {
    vector<long long> res;
    for (long long i = 1; i * i <= N; ++i) {
        if (N % i != 0) continue;
        res.push_back(i);
        if (N / i != i) res.push_back(N / i);
    }
    sort(res.begin(), res.end());
    return res;
}

int main(){
  ll t;
  cin>>t;
  while(t--){
    ll a,b;
    cin>>a>>b;
    if(a==b){
      cout<<0<<endl;
    }else{
      bool ok=false;
      vector<ll> dv=calc_divisors(b-a);
      for(ll d:dv){
        if(a<=d){
          cout<<d-a<<endl;
          ok=true;
          break;
        }
      }
      if(!ok){
        cout<<-1<<endl;
      }
    }
  }
}