using System;

public class Program

{

    public static void Main()

    {

        int i;

        Console.WriteLine("Enter a number: ");

        i=Convert.ToInt32(Console.ReadLine());

        int sum;

        while(i>0)

        {

            sum = i * 2;

            Console.WriteLine(sum);

            Console.WriteLine("Enter a number: ");

            i=Convert.ToInt32(Console.ReadLine());

        }

        Console.WriteLine("Number is less than 0. Invalid.");

    }

}