Problem:
Score : 100 points

Problem Statement

You are given positive integers X and Y.
If there exists a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, choose one such integer and print it.
If it does not exist, print -1.

Constraints

1 ≤ X,Y ≤ 10^9

X and Y are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, or print -1 if it does not exist.

Sample Input 1

8 6

Sample Output 1

16

For example, 16 is a multiple of 8 but not a multiple of 6.

Sample Input 2

3 3

Sample Output 2

-1

A multiple of 3 is a multiple of 3.

Solution in Rust (1.15.1) (judge verdict: Accepted)

#![allow(unused_imports)]

use std::cmp;
use std::cmp::{max, min, Ordering};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
use std::io::stdin;
use std::ops::*;
use std::process::exit;
use std::str::FromStr;

#[macro_export]
macro_rules! minimum {
    ($($e:expr),+) => {
        {
            let mut res = Vec::new();
            $(
                res.push($e);
            )+
            res.into_iter().min().unwrap()
        }
    };
}

pub const MOD: usize = 1000000007;

fn main() {
    let (x, y): (usize, usize) = input_two();
    if x % y == 0 {
        println!("-1");
    } else {
        println!("{}", (1..).map(|r| x * r).find(|r| r % y != 0).unwrap());
    }
}

pub fn input<T: FromStr>() -> T {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    buf.trim().parse().ok().unwrap()
}

pub fn input_vec<T: FromStr>() -> Vec<T> {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    buf.trim()
        .split_whitespace()
        .map(|x| x.parse().ok().unwrap())
        .collect::<Vec<_>>()
}

pub fn input_lines<T: FromStr>(l: usize) -> Vec<T> {
    let mut res = Vec::new();
    for _i in 0..l {
        res.push(input());
    }
    res
}

pub fn input_lines_two<T: FromStr, U: FromStr>(l: usize) -> Vec<(T, U)> {
    let mut res = Vec::new();
    for _i in 0..l {
        res.push(input_two());
    }
    res
}

pub fn input_lines_three<T: FromStr, U: FromStr, V: FromStr>(l: usize) -> Vec<(T, U, V)> {
    let mut res = Vec::new();
    for _i in 0..l {
        res.push(input_three());
    }
    res
}

pub fn input_two<T: FromStr, U: FromStr>() -> (T, U) {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    let s: Vec<&str> = buf.split_whitespace().collect();
    (
        s[0].parse::<T>().ok().unwrap(),
        s[1].parse::<U>().ok().unwrap(),
    )
}

pub fn input_three<T: FromStr, U: FromStr, V: FromStr>() -> (T, U, V) {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    let s: Vec<&str> = buf.split_whitespace().collect();
    (
        s[0].parse::<T>().ok().unwrap(),
        s[1].parse::<U>().ok().unwrap(),
        s[2].parse::<V>().ok().unwrap(),
    )
}

pub fn input_four<T: FromStr, U: FromStr, V: FromStr, W: FromStr>() -> (T, U, V, W) {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    let s: Vec<&str> = buf.split_whitespace().collect();
    (
        s[0].parse().ok().unwrap(),
        s[1].parse().ok().unwrap(),
        s[2].parse().ok().unwrap(),
        s[3].parse().ok().unwrap(),
    )
}