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: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>


int main(void)
{
	long long i, no, X, Y;
	scanf("%lld %lld", &X, &Y);

	no = 1000000000000000000 / X;

	if ( X==Y ||
		(X>Y && !(X%Y)) ){printf("-1"); return 0;}
	
	for(i=2;i<=no;i++){	
		if ((X * i) % Y){ 
			printf("\n%lld",X*i); return 0;
		}
	}
	printf("\n-1");
	return 0;
}