The Sum.

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

namespace ConsoleApp3
{
    class Program
    {
        static void Main(string[] args)
        {
            bool inputError = true; // This is used to continue the loop if the user inputs something other than Y or N.
            char userContinue; // This is for checking whether the user wishes the program to continue calculating numbers.
            bool userChoice = true; // this is used to continue the loop of calculating numbers.
            int numOne; // this is the number the user inputs.
            int sum; // this is used to calculate the sum of numbers from 0 to the input.
            while (userChoice == true)
            {
                Console.WriteLine("Please enter a whole number: ");
                numOne = Convert.ToInt32(Console.ReadLine());
                sum = 0;

                if (numOne > 0)
                {
                    for (int i = numOne; i > 0; i--)
                    {
                        sum = sum + i;
                    }
                }

                else if (numOne < 0)
                {
                    for (int i = numOne; i < 0; i++)
                    {
                        sum = sum + i;
                    }
                }

                else
                {
                    Console.WriteLine("The sum is 0.");
                }

                Console.WriteLine("The sum of all the numbers up to " + numOne + " from 0 is: " + sum);
                Console.WriteLine("Do you wish to try again? Choose Y or N.");
                inputError = true;

                while (inputError == true)
                {
                    userContinue = Console.ReadKey().KeyChar;
                    if (userContinue.Equals('Y'))
                    {
                        userChoice = true;
                        inputError = false;
                    }

                    else if (userContinue.Equals('N'))
                    {
                        userChoice = false;
                        inputError = false;
                    }

                    else
                    {
                        Console.WriteLine("Not a valid input, try again.");
                        inputError = true;
                    }
                }

            }
            Console.ReadKey();
        }
    }
}