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>

#define MAX 1000000000000000000

int main(void){
    unsigned long int x,y;
    unsigned long int i;
    scanf("%d %d", &x, &y);
    for(i = x; i <= MAX; i += x ){
        if(i % y == 0){
            printf("-1");
            return 0;
        }
        else{
            printf("%lu", i);
            return 0;
        }
    }
    printf("-1");
    return 0;
}