use std::io::Write;
use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;
type Set<T> = BTreeSet<T>;
type Deque<T> = VecDeque<T>;

fn main() {
    input! {
        l: i32,
        r: i32,
    }
    let ans = if r - l == 1 {
        -1
    } else if r == l {
        r
    } else {
        r - 1
    };
    println!("{}", ans);
}

fn test() {
    let n = 20i32;
    for r in 1..=n {
        let mut p = (1..=r).collect::<Vec<_>>();
        let mut set = Set::new();
        set.insert(p.clone());
        let mut dfs = vec![p];
        while let Some(a) = dfs.pop() {
            for i in 0..a.len() {
                for j in 0..i {
                    if (a[i] + a[j]) % 2 == 0 {
                        let mut a = a.clone();
                        let v = (a[i] + a[j]) / 2;
                        a.remove(i);
                        a.remove(j);
                        a.push(v);
                        a.sort();
                        if set.insert(a.clone()) {
                            dfs.push(a);
                        }
                    }
                }
            }
        }
        let ans = set.iter().filter(|p| p.len() == 1).map(|p| p[0]).max().unwrap_or(-1);
        println!("{}: {}", r, ans);
    }
}

// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[macro_export]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[macro_export]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------