using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    private static readonly Dictionary<int, long> Sums = new Dictionary<int, long> { { 0, 1 } };

    private static readonly Dictionary<int, long> ResultSet = new Dictionary<int, long>();

    private static readonly List<long> ValidPrimes = new List<long>();

    public static void Main()

    {

        int perfectNumber = -1;

            while (perfectNumber != 0)

            {

                Console.WriteLine("Which Perfect Number would you like me to find? (0 to exit):");

                var input = Console.ReadLine();

                if (input.Equals("0")) return;

                while (!int.TryParse(input, out perfectNumber))

                {

                    Console.WriteLine("Which Perfect Number would you like me to find? (0 to exit):");

                    input = Console.ReadLine();

                }

​

                if (perfectNumber <= 7)

                {

                    Console.WriteLine("That would be: {0}", GetPerfectNumber(perfectNumber));

                }

                else

                {

                    Console.WriteLine("Nice try buddy. How about 7 or less?");

                }

            }

        }

​

        private static long GetPerfectNumber(int sequenceNumber)

        {

            if (ResultSet.ContainsKey(sequenceNumber)) return ResultSet[sequenceNumber];

            var pow = Sums.Last().Key + 1;

            var lastPerfect = ResultSet.Any() ? ResultSet.Last().Key : 0;

            do

            {

                var result = (long)Math.Pow(2, pow + 1) - 1;

                Sums.Add(pow, result);

                if (IsPrime(result))

                {

                    ResultSet.Add(++lastPerfect, result * (long)Math.Pow(2, pow));

                }

                pow++;

            }

            while (!ResultSet.ContainsKey(sequenceNumber));

​

            return ResultSet[sequenceNumber];

        }

​

        private static bool IsPrime(long candidate)

        {

            if (ValidPrimes.Contains(candidate)) return true;

​

            if ((candidate & 1) == 0)

            {

                if (candidate == 2)

                {

                    ValidPrimes.Add(candidate);

                    return true;

                }

                return false;

            }

​

            for (int i = 3; (i * i) <= candidate; i += 2)

            {

                if ((candidate % i) == 0 || i * i < 0)

            {

                    return false;

                }

            }

            if(candidate != 1) ValidPrimes.Add(candidate);

​

            return candidate != 1;

        }

}