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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace apc001a
{
    public class Problem
    {
        private int X_, Y_;
        public Problem(int x, int y) {
            X_ = x;
            Y_ = y;
        }
        public int Solve() {
            var list = new List<int>() { X_ };
            //checked {
                for(var i = 2; X_ * i <= (int)Math.Pow(10, 18); ++i) {
                    list.Add(X_ * i);
                }
            //}
            var v = list.Where(k => k % Y_ != 0);
            return (v.Any()?v.First():-1);

        }
    }
    class Program
    {
        static void Main(string[] args) {
            var x_y = Console.ReadLine().Split();
            var X = int.Parse(x_y[0]);
            var Y = int.Parse(x_y[1]);
            var problem = new Problem(X, Y);

            Console.WriteLine(problem.Solve());
        }
    }
}