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)

import sys
import copy
import math
import bisect
import pprint
import bisect
from functools import reduce
from copy import deepcopy
from collections import deque

def lcm(x, y):
    return (x * y) // math.gcd(x, y)

if __name__ == '__main__':
    a = [int(i) for i in input().split()]

    for i in range(1,10000000002):
        if a[0] // a[1]==0:
            print(-1)
            break
        
        if i ==10000000001:
            print(-1)
            break
        if a[0]*i %a[1] >0:
            print(a[0]*i)
            break
        if a[0]*i >10000000001:
            print(-1)
            break