#[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 && b != c && c != a && (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 small = std::cmp::min(a, b);
    let big = std::cmp::max(a, b);

    if beautiful(small, big, big - small) {
        println!("{}", big - small);
    } else if beautiful(small, big, small + big) {
        println!("{}", small + big);
    } else {
        println!("-1");
    }

}

fn main() {
    solve();
}