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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
#include<math.h>
int main()
{
   int x,y,a=0;
   scanf("%d%d",&x,&y);
   if(x%y==0||y%x==0)
   {
       printf("-1\n");
       return 0;
   }
   else
    {
        a=x;
   while(1)
   {
       a++;
       if(a%x==0&&a%y!=0)
       {
           printf("%d\n",a);
           break;
       }
   }
   }
    return 0;
}