fn gcd(a: usize, b: usize) -> usize {
    if b == 0 {
        a
    } else {
        gcd(b, a % b)
    }
}

fn run() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    let mut it = s.trim().split_whitespace();
    let a: usize = it.next().unwrap().parse().unwrap();
    let b: usize = it.next().unwrap().parse().unwrap();
    if gcd(a, b) > 1 {
        println!("-1");
    } else {
        let mut ans = a * b;
        for x in 1..=(a * b) {
            for i in 1..=(x / a) {
                if (x - i * a) % b == 0 {
                    ans -= 1;
                    break;
                }
            }
        }
        println!("{}", ans);
    }
}

fn main() {
    run();
}