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 OCaml (4.02.3) (judge verdict: Memory Limit Exceeded)

let () =
  let (x,y) = Scanf.scanf "%d %d\n" (fun x y -> x,y) in
  let y'=int_of_float (sqrt (float_of_int y)) in
  let rec min_yp i =
    if i > x || i > y' then 2
    else if y mod i <> 0 then i
    else min_yp i+1
  in
  let check  =
    if x mod y = 0 then -1
    else if x>y then x
    else x * (min_yp 2)
  in
  let ans = check in
  print_int ans;
  
;;