Score : 100 points
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.
Input is given from Standard Input in the following format:
X Y
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.
8 6
16
For example, 16 is a multiple of 8 but not a multiple of 6.
3 3
-1
A multiple of 3 is a multiple of 3.