use proconio::input;

#[proconio::fastout]
fn main() {
    input! {
        t: usize,
    }

    for _ in 0..t {
        input! {
            a: usize,
            b: usize,
        }

        if let Some(result) = solve(a, b) {
            println!("{}", result);
        } else {
            println!("-1");
        }
    }
}

fn solve(a: usize, b: usize) -> Option<usize> {
    if a == b {
        return Some(0);
    }

    let list = divisors(b - a);

    for divisor in list {
        if divisor >= a {
            return Some(divisor - a);
        }
    }

    None
}

fn divisors(n: usize) -> Vec<usize> {
    let mut list = Vec::new();

    {
        let mut i = 1;

        while i * i <= n {
            if n % i == 0 {
                list.push(i);

                if n / i != i {
                    list.push(n / i);
                }
            }

            i += 1;
        }
    }

    list.sort();

    list
}