Sum big numbers

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

namespace _10.Sum_big_numbers
{
    class Program
    {
        static void Main(string[] args)
        {
            StringBuilder sum = new StringBuilder("");
            var firstNum = Console.ReadLine();
            var secondNum = Console.ReadLine();
            var first = "";
            var second = "";

                if (firstNum[0] == '0' || secondNum[0] == '0')
            {
                first = firstNum.TrimStart(new char[] { '0' });
                second = secondNum.TrimStart(new char[] { '0' });
                firstNum = first;
                secondNum = second;
            }


            //if (firstNum[0] == '0' || secondNum[0] == '0')
            //{
            //    if (firstNum[0] == '0')
            //    {
            //        var length = firstNum.Length;
            //        var flagIsZero = true;
            //        var count = 0;
            //        for (int i = 0; i < length; i++)
            //        {
            //            if (flagIsZero)
            //            {
            //                if (firstNum[i] == '0')
            //                {
            //                    count++;
            //                }
            //                else
            //                {
            //                    flagIsZero = false;
            //                }
            //            }
            //        }
            //        first = firstNum.Remove(0, count);
            //    }

            //    if (secondNum[0] == '0')
            //    {
            //        var flagIsZero = true;
            //        var length = secondNum.Length;
            //        var count = 0;
            //        for (int i = 0; i < secondNum.Length; i++)
            //        {
            //            if (flagIsZero)
            //            {
            //                if (secondNum[i] == '0')
            //                {
            //                    count++;
            //                }
            //                else
            //                {
            //                    flagIsZero = false;
            //                }
            //            }
            //        }
            //        second = secondNum.Remove(0, count);
            //    }

            //}
            //Console.WriteLine(first);
            //Console.WriteLine(second);

            if (firstNum.Length > secondNum.Length)
            {
                var difference = firstNum.Length - secondNum.Length;
                secondNum = new string('0', difference) + secondNum;

            }
            else if (firstNum.Length < secondNum.Length)
            {
                var difference = secondNum.Length - firstNum.Length;
                firstNum = new string('0', difference) + firstNum;

            }

            //Console.WriteLine(firstNum);
            //Console.WriteLine(secondNum);
            var leftover = 0;

            for (int i = secondNum.Length - 1; i >= 0 ; i--)
            {

                var sumNums = Convert.ToInt32(firstNum[i] - '0') + Convert.ToInt32(secondNum[i] - '0') + leftover;
                if (sumNums < 10)
                {
                    sum.Append(sumNums);
                    leftover = 0;
                }
                else
                {
                    if (i != 0)
                    {
                        if (Convert.ToInt32(firstNum[i] - '0') + Convert.ToInt32(secondNum[i] - '0') + leftover == 10)
                        {
                            sum.Append('0');
                            leftover = 1;
                        }
                        else
                        {

                            sum.Append(sumNums - 10);
                            leftover = 1;
                            //Console.WriteLine(leftover);

                        }


                    }
                    else
                    {
                        sum.Append(string.Join("", Convert.ToString(Convert.ToInt32(firstNum[i] - '0') + Convert.ToInt32(secondNum[i] - '0') + leftover).Reverse()));


                    }

                }
                //Console.WriteLine(sum);
            }
            Console.WriteLine(string.Join("", sum.ToString().Reverse()));
        }
    }
}