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: Runtime Error)

    using System;
    using System.Linq;
     
    namespace apc001_b
    {
        class Program
        {
            static void Main(string[] args)
            {
                int n = int.Parse(Console.ReadLine());
                long[] a = Console.ReadLine().Split().Select(long.Parse).ToArray();
                long[] b = Console.ReadLine().Split().Select(long.Parse).ToArray();
                long ret1 = 0; long ret2 = 0;
                for (int i = 0; i < n; i++)
                {
                    if (a[i] > b[i]) ret1 += a[i] - b[i];
                    else ret2 += (b[i] - a[i] + 1) / 2;
                }
                Console.WriteLine(Math.Max(ret1,ret2) <= b.Sum() - a.Sum() ? "Yes" : "No");
            }
        }
    }