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 C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;

namespace moff1 {
    class MainClass {
        static long max = 1000000000000000000L;
        public static void Main(string[] args) {
            // 以下の2文が加わるだけです！
            //var exStdIn = new System.IO.StreamReader("stdin.txt");
            //System.Console.SetIn(exStdIn);

            // logic
            var line = Console.ReadLine();
            var x = int.Parse(line.Split(' ')[0]);
            var y = int.Parse(line.Split(' ')[1]);
            int ans = -1;

            var a = Math.Max(x, y);
            var b = Math.Min(x, y);
            if (a / b != 0) {
                for (var i = 1; i <= max; i++) {
                    if (i != x && i % x == 0) {
                        if (i % y != 0) {
                            ans = i;
                            break;
                        }
                    }
                }
            }

            Console.WriteLine(ans);
        }
    }
}