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 Go (1.6) (judge verdict: Runtime Error)

///
// File:  a.go
// Author: ymiyamoto
//
// Created on Sat Feb  3 23:01:46 2018
//
package main

import (
	"fmt"
)

func main() {
	var X, Y int

	if X%Y == 0 {
		fmt.Println(-1)
		return
	}

	for i := 0; ; i++ {
		if X*i%Y != 0 {
			fmt.Println(X)
			return
		}
	}
}