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 Pascal (FPC 2.6.2) (judge verdict: Wrong Answer)

{var n,m,ans:int64;
var a:int64;
function gys(a,b:int64):int64;
Var
  c:int64;
begin
  repeat
    c:=a mod b;
    a:=b;
    b:=c;
  until c=0;
  exit(a);
end;
begin
  read(n,m);
  if m mod n=0 then begin writeln(-1);exit;end;
  ans:=n*gys(n,m);if ans<=1000000000000000000
end.        }
var n,m,ans:int64;
var i:longint;
var a:int64;
begin
  read(n,m);
  if m mod n=0 then begin writeln(-1);exit;end;
  for i:=2 to 2147483647 do
  begin
    a:=n*i;
    if a>1000000000000000000 then begin writeln(-1);exit;end;
    if a mod m>0 then begin writeln(a);exit;end;
  end;
  writeln(-1);
end.