6. Sum big numbers

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


class Program
{
    static void Main(string[] args)
    {
        string n1 = Console.ReadLine().TrimStart(new char[] { '0' });
        string n2 = Console.ReadLine().TrimStart(new char[] { '0' });
        StringBuilder result = new StringBuilder();
        if (n1.Length > n2.Length)
        {
            n2 = n2.PadLeft(n1.Length, '0');
        }
        else if (n1.Length < n2.Length)
        {
            n1 = n1.PadLeft(n2.Length, '0');
        }
        char[] str1 = n1.ToCharArray();
        char[] str2 = n2.ToCharArray();

        sbyte remainder = 0;
        sbyte addition = 0;
        for (int i = str1.Length - 1; i >= 0; i--)
        {
            sbyte num1 = sbyte.Parse(str1[i].ToString());
            sbyte num2 = sbyte.Parse(str2[i].ToString());
            num1 += addition;
            addition = 0;
            if (num1+num2<10)
            {
                result.Append(num1 + num2);
            }
            else
            {
                remainder = (sbyte)((num1 + num2) % 10);
                result.Append(remainder);
                addition= (sbyte)((num1 + num2) /10);
            }
        }
        if (addition!=0)
        {
            result.Append(addition);
        }

        char[] endResult=result.ToString().ToCharArray();
        Array.Reverse(endResult);

        Console.WriteLine(string.Join("",endResult));
    }
}