Task.05.Сума от цифри от ляво на дясно

using System;
using System.Collections.Generic;
class SumOfDigits
{
    static long[] numbers = new long[18];
    static int startNum, endNum;
    static long result = 0;
    static List<long> list = new List<long>();
    static void Main()
    {
        Console.WriteLine("Write a number >= 0  and <= 100000000000000000");
        Console.Write("number = ");
        long number;
        bool isCorrect = long.TryParse(Console.ReadLine(), out number);

        if(!isCorrect || number > 100000000000000000
            || number < 0 || (number / 100) == 0)
        {
            Console.WriteLine("Incorrect input! Try again");
            Main();
        }
        CheckedCorrectDigits(number);

        long sum = 1;
        for (int i = 0; i < 18; i++)//добавян числата с които деля
        {
                numbers[i] = sum;
                sum *= 10;
        }
        Recursion(number);
        Console.WriteLine("Sum of the numbers: " + result);
    }

    private static void CheckedCorrectDigits(long number)
    {
        Console.Write("Start digit = ");
        startNum = int.Parse(Console.ReadLine());
        string lenghtStartNum = "" + number;
        Console.Write("Last digit = ");
        endNum = int.Parse(Console.ReadLine());
        string lenghtEndNum = "" + number;

        if (lenghtEndNum.Length < endNum)
        {
            Console.WriteLine("Write smaller last digit!");
            CheckedCorrectDigits(number);
        }
        if(lenghtStartNum.Length < startNum)
        {
            Console.WriteLine("Write smaller start digit!");
            CheckedCorrectDigits(number);
        }
        if(startNum > endNum)
        {
            Console.WriteLine("Start digit must be smaller or equal to end digit!");
            CheckedCorrectDigits(number);
        }
    }
    private static void Recursion(long num)
    {
        list.Insert(0, num % 10);
        if(num / numbers[startNum] == 0)//използвам индекса, ако е нула значи  сме на тази цифра
        {
            for (int i = 0; i <= endNum - startNum; i++)
            {
                result += list[i];
            }
            return;
        }
        Recursion(num / 10);
    }
}