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: Time Limit Exceeded)

X, Y = map(int, input().split(' '))

i = 1
cond = False
if X != Y:
    while i <= 1000000000:
        if (i % X) == 0:
            if (i % Y) != 0:
                cond = True
                break
        i += 1

if cond:
    print(i)
else:
    print(-1)