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>
#define IMAX 100000

int main()
{
  int x,y;
  scanf("%d %d",&x,&y);
  if (!(x%y)) {
    printf("-1\n");
    return 0;
  }
  for(int i=1;i<IMAX;i++){
    int k=x*i;
    if(k%y!=0){
      printf("%d\n",k); break;
    }

  }
  return 0;
}