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: Accepted)

#include <stdio.h>
#include <math.h>
#define Max 1000000000000000000
/*long long gcd(long long a,long long b)
{
	if(b==0) return a;
	return gcd(b,a%b);
}*/
int main()
{
	unsigned long long x,y,i,f;
	scanf("%llu%llu",&x,&y);
	if(x%y)
	{
		f=0;
		for(i=2;i<=Max/x;i++)
		{
			if(i*x%y!=0)
			{
				f=1;
				printf("%llu\n",i*x);
				break;
			}			
		}
		if(f==0)
			printf("-1\n");
	}
	else
	{
		printf("-1\n");
	}
	return 0;
}