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 Python (3.4.3) (judge verdict: Wrong Answer)

x,y=map(int,input().split())
a=0
for i in range(min(x,y),max(x,y)):
    if x%y==0 or y%x==0:
        print(-1)
        a=1
        break
    if i%x==0 and i%y!=0:
        print(i)
        a=1
        break
if a==0:
    print(-1)