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: Time Limit Exceeded)

var
    n,m:longint;
    i:qword;
begin
    readln(n,m);
    if(n=m)then
    begin
        writeln(-1);
        exit;
    end;
    while(i<1000000000000000000)do
    begin
        inc(i);
        if(i mod n=0)and(i mod m<>0)then
        begin
            writeln(i);
            exit;
        end;
    end;
end.