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: usize = gcd(max(a, b), min(a, b));
    if gcd > 1 {
        println!("{}", -1);
    } else {
        let lcm: usize = a * b;
        let aa: Vec<usize> = (0..=b).map(|i| a*i).collect();
        let bb: Vec<usize> = (0..=a).map(|i| b*i).collect();
        let mut abs: HashSet<usize> = HashSet::new();
        for i in aa.iter() {
            for j in bb.iter() {
                if i + j <= lcm {
                    abs.insert(i+j);
                }
            }
        }
        let result: usize = lcm + 1 - abs.len();
        println!("{}", result);
    }
}