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>
#define max	1000000000000000000
int main()
{
	long long x,y,z;
	scanf("%lld%lld",&x,&y);
	z=x+x;
	if(x==y||x%y==0||y==1)
	{
		printf("-1\n");
		return 0;
	 } 
	 if(x==1)
	{
			printf("%lld\n",y+1);
		return 0;
	}
	while(z<=max&&z>=1&&(((z%y))==0))
	{
		z+=x;
	}
	if(z<=max&&z>=1&&((z%y)==0))
		printf("%lld\n",z);
	else 
		printf("-1\n");

	return 0;
}
//x*z这个数%y！=0