use std::collections::HashSet;
use std::cmp::{max, min};

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

fn main() {
    let mut ab: String = String::new();
    std::io::stdin().read_line(&mut ab).ok();
    let ab: Vec<usize> = ab.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let a: usize = ab[0];
    let b: usize = ab[1];

    let gcd_val = gcd(max(a, b), min(a, b));
    if gcd_val > 1 {
        println!("{}", -1);
    } else {
        let lcm: usize = a * b / gcd_val;
        let mut used: HashSet<usize> = HashSet::new();
        for i in 0..b {
            for j in 0..a {
                if i * a + j * b < lcm {
                    used.insert(i * a + j * b);
                }
            }
        }
        println!("{}", lcm - used.len());
    }
}