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

#define ll long long int
#define str string
#define forn(a, b) for (int i = a; i < b; i++)
#define forj(a, b) for (int j = a; j < b; j++)
#define fork(a, b) for (int k = a; k < b; k++)
#define forr(a, b) for (int i = a; i >= b; i--)
#define forrj(a, b) for (int j = a; j >= b; j--)
#define forrk(a, b) for (int k = a; k >= b; k--)
#define yn(x) (x ? "YES" : "NO")
#define pb push_back
#define pop pop_back
#define bara INT_MAX
#define chota INT_MIN
#define bohotbara LLONG_MAX
#define bohotchota LLONG_MIN
#define all(x) x.begin(), x.end()

// def chck(a, b):
//     flg = True
//     for i in range(a, b, -1):
//         for j in range(b, 0, -1):
//             # print(i, j)
//             if i%j==0 and i//j <= b:
//                 break
//             if i%j==0 and i//j > b:
//                 print(i)
//                 return
//     print(-1)

// t = int(input())
// for x in range(t):
//     a, b= map(int, input().split())
//     if a==b: print(-1)
//     else: chck(a,b)

void chck(ll a, ll b){
    forr(a, 1){
        forrj(b, 1){
            if(i%j==0 && i/j <=b){
                break; 
            }
            if(i%j==0 && i/j>b){
                cout<<i<<endl; 
                return ;
            }
        }
    }
    cout<<-1<<endl; 
}

void solve()
{
    ll t;
    cin >> t;
    while (t--)
    {
        ll a, b; cin>>a>>b; 
        if(a==b) cout<<-1<<endl; 
        else chck(a,b); 
    }
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    solve();
}