#[allow(unused_macros)]
macro_rules! invec {
    ( $t:ty ) => {{
        let mut s = String::new();
        match std::io::stdin().read_line(&mut s) {
            Ok(0) => Vec::<$t>::new(),
            Ok(n) => s.trim().split_whitespace().map(|s| s.parse::<$t>().unwrap()).collect::<Vec<$t>>(),
            Err(_) => Vec::<$t>::new(),
        }
    }}
}

#[allow(unused_macros)]
macro_rules! input {
    ( $($t:ty),* ) => {{
        let mut s = String::new();
        std::io::stdin().read_line(&mut s);
        let mut splits = s.trim().split_whitespace();
        ($(
            {
                 splits.next().unwrap().parse::<$t>().unwrap()
            },
        )*)
    }}
}

fn beautiful(a: i64, b: i64, c: i64) -> bool {
    (a + b) % c == 0 && (b + c) % a == 0 && (c + a) % b == 0
}

#[allow(unused_must_use)]
#[allow(unused_variables)]
fn solve() {
    let (a, b) = input!(i64, i64);

    let a = std::cmp::min(a, b);
    let b = std::cmp::max(a, b);

    let min = std::cmp::max(2 * a - b, 1) as usize;
    let max = (a + b) as usize;

    for i in min..=max {
        if i as i64 != a && i as i64 != b {
            if beautiful(a, b, i as i64) {
                println!("{}", i);
                std::process::exit(0);
            }
        }
    }

    println!("-1");

}

fn main() {
    solve();
}