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 JavaScript (node.js v5.12) (judge verdict: Time Limit Exceeded)

'use strict'

function main(s) {

  s = s.split('\n')[0].split(' ').map(n => Number(n));

  const a = s[0];
  const b = s[1];
  let c = a;
  let ans = -1;

  while(c <= 1e18) {
    if (c % a === 0 && c % b !== 0) {
      ans = c;
      break;
    }
    c += 1;
  }

  console.log(c);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));