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: Time Limit Exceeded)

use std::io::stdin;

fn main() {
    let base : u64 = 10;
    let scan = stdin();
    let mut line = String::new();

    let _ = scan.read_line(&mut line);

    let mut iter = line.split_whitespace();
    let x : u64 = iter.next().expect("Iteration Error?").trim().parse().expect("Number pls...");
    let y : u64 = iter.next().expect("Iteration Error?").trim().parse().expect("Number pls...");

    if x == y {
        println!("{}", -1);
        return;
    }

    let mut i : u64 = 1;
    loop {
        let mul : u64 = x * i;
        i += 1;

        if mul >= base.pow(18) {
            println!("{}", -1);
            return;
        }

        if mul % y != 0 {
            println!("{}", mul);
            return;
        }
    }
}