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 Ruby (2.3.3) (judge verdict: Wrong Answer)

a, b = gets.split.map &:to_i
l = a.lcm(b)
g = a.gcd(b)
if a % b == 0then
  p -1
else
  n = a * g
  check = true
  while n % b == 0 do
    n *= g
    if l < n then
      check = false
      break
    end
  end
  p (check ? n : -1)
end