// -*- coding:utf-8-unix -*-
// #![feature(map_first_last)]
#![allow(dead_code)]
#![allow(unused_imports)]
#![allow(unused_macros)]
use std::collections::*;
use std::convert::*;
use std::convert::{From, Into};
use std::fmt::Debug;
use std::fs::File;
use std::io::prelude::*;
use std::io::*;
use std::marker::Copy;
use std::mem::*;
use std::ops::Bound::*;
use std::ops::{Add, Mul, Neg, Sub};
use std::str;
use std::vec;
use std::{cmp, process::Output};
use std::{cmp::Ordering, env::consts::DLL_PREFIX};
use std::{cmp::Ordering::*, f32::consts::PI};

const INF: i64 = 1223372036854775807;
const UINF: usize = INF as usize;
const FINF: f64 = 122337203685.0;
const INF128: i128 = 1223372036854775807000000000000;
const LINF: i64 = 2147483647;
const MOD: i64 = 1000000007;
// const MOD: i64 = 998244353;
// const MOD: i64 = INF;

const UMOD: usize = MOD as usize;
use std::cmp::*;
use std::collections::*;
use std::io::stdin;
use std::io::stdout;
use std::io::Write;

macro_rules! p {
    ($x:expr) => {
        println!("{}", $x);
    };
}
macro_rules! d {
    ($x:expr) => {
        println!("{:?}", $x);
    };
}
macro_rules! dd {
    (x:expr) => {
        dbg!(x);
    };
}

// use str::Chars;

// use str::Chars;
#[allow(dead_code)]
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

#[allow(dead_code)]
fn readi() -> (i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    iter.next().unwrap().parse::<i64>().unwrap()
}

#[allow(dead_code)]
fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}
#[allow(dead_code)]
fn read_vec2<T: std::str::FromStr>(n: u32) -> Vec<Vec<T>> {
    (0..n).map(|_| read_vec()).collect()
}

#[allow(dead_code)]
fn readii() -> (i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}

fn readff() -> (f64, f64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<f64>().unwrap(),
        iter.next().unwrap().parse::<f64>().unwrap(),
    )
}

#[allow(dead_code)]
fn readiii() -> (i64, i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}
#[allow(dead_code)]
fn readuu() -> (usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}

fn readcc() -> (char, char) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<char>().unwrap(),
        iter.next().unwrap().parse::<char>().unwrap(),
    )
}

#[allow(dead_code)]
fn readuuu() -> (usize, usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}

#[allow(dead_code)]
fn readuuuu() -> (usize, usize, usize, usize) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
        iter.next().unwrap().parse::<usize>().unwrap(),
    )
}

fn readiiii() -> (i64, i64, i64, i64) {
    let mut str = String::new();
    let _ = stdin().read_line(&mut str).unwrap();
    let mut iter = str.split_whitespace();
    (
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
        iter.next().unwrap().parse::<i64>().unwrap(),
    )
}

/// Equivalent to std::lowerbound and std::upperbound in c++
fn prime_factorization(x: usize) -> BTreeMap<usize, usize> {
    let mut res: BTreeMap<usize, usize> = BTreeMap::new();
    let mut xx = x;
    let mut p: usize = 2;
    while p * p <= xx {
        while xx % p == 0 {
            // println!("{:?}", p);
            let t = res.get_mut(&p);
            if t.is_none() {
                res.insert(p, 1);
            } else {
                *t.unwrap() += 1;
            }
            xx /= p;
        }
        // println!("{:?} {:?}", p, res);
        p += 1;
    }

    if xx != 1 {
        let t = res.get_mut(&xx);
        if t.is_none() {
            res.insert(xx, 1);
        } else {
            *t.unwrap() += 1;
        }
    }
    res
}

pub fn vector_lcm(vec: &Vec<usize>) -> usize {
    let mut res = 1;
    let mut alcm: BTreeMap<usize, usize> = BTreeMap::new();
    for i in 0..vec.len() {
        let pf = prime_factorization(vec[i]);
        // println!("{:?}", vec[i]);
        // println!("{:?}", pf);
        for i in pf {
            let t = alcm.get_mut(&i.0);
            if t.is_none() {
                alcm.insert(i.0, i.1);
            } else {
                let val = *t.unwrap();
                *alcm.get_mut(&i.0).unwrap() += max(0, i.1);
            }
        }
    }
    for i in alcm {
        res *= (i.1 + 1);
        res %= MOD as usize;
        // println!("{:?}", i);
    }

    res
}

fn solve() {
    let (n, m) = readuu();
    if m >= n {
        println!("{:?}", 1);
        return;
    }
    if (n % 2 == 1) || (n > m * 2) {
        println!("{:?}", -1);
        return;
    }
    println!("{:?}", 2);
    return;
}

fn main() {
    solve();
}