Sum Big Numbers

using System;
using System.Text;

class SumBigNumbers
{
    static void Main(string[] args)
    {
        string firstWord = Console.ReadLine();
        string secondWord = Console.ReadLine();

        Sum(firstWord, secondWord);
    }

    static void Sum(string firstWord, string secondWord)
    {
        StringBuilder builder = new StringBuilder();

        int maxLenth = Math.Max(firstWord.Length, secondWord.Length);
        int remainder = 0;

        firstWord = firstWord.PadLeft(maxLenth, '0');
        secondWord = secondWord.PadLeft(maxLenth, '0');

        for (int index = maxLenth - 1; index >= 0; index--)
        {
            int sum = firstWord[index] - 48 + secondWord[index] - 48 + remainder;
            builder.Insert(0, sum % 10);
            remainder = sum / 10;

            if (index == 0)
            {
                builder.Insert(0, remainder);
            }
        }

        string result = string.Join("", builder.ToString().TrimStart('0'));
        Console.WriteLine(result);
    }
}