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 N 1000000000000000000

int main(void)
{
  int X,Y;

  scanf("%d",&X);
  scanf("%d",&Y);

  if(X % Y != 0){
    for( int i = 0 ; i <= N ; i++ ){
      if((i % Y != 0)&&(i >= X)&&(i >= Y)){
        if(i % X == 0){
          printf("%d",i);
          break;
        }
      }
    }
  }
  else{
    printf("%d",-1);
  }

  return 0;

}