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)

/* for vjudge contest anu-cpc a */
#include <stdio.h>

#define MAX 1000000000000000000

int main(int argc, char** argv)
{
    long long x, y;
    long long i;
    
    scanf("%ld %ld", &x, &y);
    if (x % y == 0)
    {
        printf("-1\n");
        return 0;
    }

    for (i = 2; i < MAX / x; ++i)
    {
        if (x * i % y)
        {
            printf("%ld\n", x * i);
            return 0;
        }
    }

    printf("-1\n");
    return 0;
}