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 C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>

int a, b, c, d, e;

int main()
{
	std::cin >> a;
	std::cin >> b;

	d = 100000000000000000 / a;
	for(int i = 1 ; i <= d; i++)
	{
		e =  a * i; 
		if ( a == b)
		{
			c = -1;
			break;
		}else if((e % b != 0))
		{
			c = e;
			break;
		}
		else{
			c = -1;
		}
	}
	std::cout << c << std::endl;
}