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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
#include<math.h>

int main()
{
     long long i, j, X, Y;
     scanf("%lld %lld",&X,&Y);
     j = 1;
     if(X%Y==0||Y%X==0)
        printf("-1");
     else
     {
         for(i=X;i<=pow(10,18);i=X*j)
         {
             j++;
             if(i>X&&i%Y!=0)
             {
                 printf("%lld",i);
                 break;
             }

         }
         if(i>pow(10,18))
            printf("-1");
     }
     return 0;
}