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)

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

long long gcd(long long x, long long y)
{
    long long r;
    while ((r = x % y) != 0) {
        x = y;
        y = r;
    }
    return y;
}

long long lcm(long long  x, long long y)
{
    return (x * y / gcd(x, y));
}

int main() {
	long long x, y, l;
	long long i, a;
	scanf("%lld %lld", &x, &y);
	a = -1;
	l = lcm(x, y);

	for (i = x; i <= l; i += x) {
		if (i % y != 0) {
			a = i;
			break;
		}
	}
	if (a > 100000000000000000) {
		a = -1;
	}

	printf("%lld", a);
	return 0;
}