using System;

public class Program

{

    public static void Main(string[] args)

    {

        decimal maxDiff = 0;

        for (decimal questionPrice = 20; questionPrice <= 50; questionPrice+=0.001M) 

        {

            for (decimal expertPayout = 5; expertPayout < 20; expertPayout+=0.5M)

            {

                decimal diff = CheckPayoutRate(questionPrice, expertPayout);

                if (diff > maxDiff)

                {

                    maxDiff = diff;

                }

            }

        }

        Console.WriteLine(maxDiff);

    }

    public static decimal CheckPayoutRate(decimal questionPrice, decimal payout)

    {

        decimal rate = payout / questionPrice;

        rate *= (decimal) Math.Pow(10, 3);

        rate = Math.Ceiling(rate);

        rate = rate * (decimal) Math.Pow(10, -1 * 3);

        return (rate * questionPrice) - payout;

    }

}