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: Runtime Error)

def main():
    _ = input()
    a_list = map(int, input().split())
    b_list = map(int, input().split())
    diffs = map(lambda a, b: b - a, a_list, b_list)
    total = 0
    count = 0
    for diff in diffs:
        if diff > 0:
            count += diff // 2
            total += diff % 2
        else:
            count += -diff
            total += diff
    if count <= total:
        print("Yes")
    else:
        print("No")


if __name__ == '__main__':
    main()