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)

x, y = gets.chomp.split(" ").map(&:to_i)

if (x == y) || (y == 1) then
  puts -1
  exit
end

i = 1
while true
  n = x * i
  if n >= 10**8 then
    puts -1
    exit
  elsif n % y != 0
    puts n
    exit
  end
  i += 1
end