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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

class Main {
   static long gcd(long a, long b)
   {
      if(a % b == 0){
         return(b);
         }
      return(gcd(b, a % b));
   }

   public static void main(String[] args)
   {
      Scanner scan = new Scanner(System.in);
      long    X    = scan.nextLong();
      long    Y    = scan.nextLong();
      int     a    = -1;

      if(X % Y == 0){
         System.out.println(a);
         return;
         }

      for(int i = 2;; i++){
          if(Y % i != 0 && gcd(Y, i) == 1){
             System.out.println(i * X);
             return;
             }
          }
   }
}