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  <bits/stdc++.h>
using  namespace  std;

int main()

{
    long long int  x,y;
	
    cin>>x>>y;
     
    int i=x; 
		
	while(i<=100000000*x)
	  {  
	    i+=x; 
	  	if(i%x==0  &&  i%y!=0)
	   	  { 
	   	     cout<<i<<endl;
	   	     break;
		  }	
	  
	  }
	  
	cout<<"-1"<<endl;
	     
	return 0;
}