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>
long int pow(void);

long int pow()
{
	long int no = 10;
	int i;
	for(i = 0; i < 18; i++)
		 	no = no*no;
			 
	return no; 
}

int main()
{
long int x, y;
scanf("%ld %ld", &x, &y);
if(x == y)
{
	printf("-1");
	return 0; 
}
long int n = pow();
int k = 2,i, flag = 0;
for(i = 0; i < 100; i++)
{
	long int no = x * k;
	if(no % y != 0)
	{
		printf("%ld", no);
		break;
	}
	if(no >= n)
	{
		printf("-1");
		break;
	}
	k++;
}
return 0;
}