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 <stdint.h>

int main(void)
{
    uint32_t X, Y;
    int32_t answer;
    scanf("%u %u", &X, &Y);
    if (Y == 1 || X == Y) {
        answer = -1;
    } else {
        uint32_t x = X;
        while (x < Y) x += X;
        while (X <= 1000000000000000000) {
            if (x % Y != 0) {
                answer = x;
                break;
            }
            x += X;
        }
    }
    printf("%d\n", answer);
    return 0;
}