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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <string.h>
#include <math.h>
int main(void)
{
	 long long a,b,t,i;
	 long long ans;
	 int tamp;
	 while(scanf("%lld %lld",&a,&b)!=EOF)
	 {
	 	tamp=1;
		 if(a%b==0||b%a==0)
	 	{
	 		printf("-1\n");
	 		continue;
		 }
		for(i=2;ans<pow(10,18);i++)
		{
			ans=i*a;
			if(ans%b!=0)
			{
				printf("%lld\n",ans);
				tamp=0;
				break;
			}
			
		}
		if(tamp==0)
		printf("-1\n");
		
	  } 
}