use std::cmp::max;
use std::io;
use std::io::{Write, BufRead};

pub fn main(){
    let mut INPUT = io::BufReader::new(io::stdin().lock()).lines().map(Result::unwrap);
    let mut OUTPUT = io::BufWriter::new(io::stdout().lock());
    let test_cases:u32 = INPUT.next().unwrap().parse().unwrap();

   for _ in 0..test_cases{
    let inp = INPUT.next().unwrap();
    let mut inpit = inp.split_ascii_whitespace();
    let a:u32 = inpit.next().unwrap().parse().unwrap();
    let b:u32 = inpit.next().unwrap().parse().unwrap();

    let mut x = None;

    if a == b{x = Some(0);}

    for k in 2..=b/a {
        if (b-k*a) % (k-1) == 0{
            x = Some((b-k*a) / (k-1))
        }
    }

    match x {
        None => writeln!(OUTPUT, "-1").unwrap(),
        Some(x) => writeln!(OUTPUT, "{}", x).unwrap()
    }

   }
}