#include<bits/stdc++.h>
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define pb push_back
#define ll long long int
#define yes cout<<"YES\n"
#define no cout<<"NO\n"
#define mx_ele(v) *max_element(v.begin(),v.end())
#define mnm_ele(v) *min_element(v.begin(),v.end())
#define endl "\n"
using namespace std;
void smile()
{
    int m,n;
    cin>>m>>n;
    int bound=min(m,n);
    if(m%2==0 and n%2!=0)
        cout<<"-1\n";
    else if(m%2!=0 and n%2==0)
        cout<<"-1\n";

    else
    {
        if(m%2==0)
        {
            if(m<2 or n<2)
                cout<<"-1\n";
            else
                cout<<2<<endl;
        }
        else if(m%2!=0)
        {
            if(m<3 or n<3)
                cout<<"-1\n";
            else
                {
                if(m%3!=0 or n%3!=0)
                cout<<"-1\n";
                else
                cout<<3<<endl;
                }
        }
    }
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int T=1;
    cin>>T;
    while(T--)
    {
        smile();
    }

}