use std::io::*;

fn gcd(a: u64, b: u64) -> u64 {
    if b == 0 {
        return a;
    }
    gcd(b, a % b)
}

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let mut itr = s.trim().split_whitespace();
    let a: u64 = itr.next().unwrap().parse().unwrap();
    let b: u64 = itr.next().unwrap().parse().unwrap();
    let c = gcd(a, b);
    if c != 1 {
        println!("-1",)
    } else {
        let mut ans = a / c * b;
        let lcm = a / c * b;
        for x in 0..b {
            for y in 0..a {
                if a * x + b * y <= lcm {
                    ans -= 1;
                }
            }
        }
        println!("{}", ans);
    }
}