StringMultiplicationOfTwoNumbers

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

namespace SumBigNumbersSecondTry
{
    public class SumBigNumbersSecondTry
    {
        public static void Main()
        {
            string firstInput = ReverseStringNumber(Console.ReadLine().TrimStart('0'));
            string secondInput = ReverseStringNumber(Console.ReadLine().TrimStart('0'));

            StringBuilder result = new StringBuilder();

            for (int firstSymbol = 0; firstSymbol < firstInput.Length; firstSymbol++)
            {
                int firstNumber = int.Parse(firstInput[firstSymbol].ToString());

                StringBuilder currSum = new StringBuilder();
                int remainder = 0;
                for (int secondSymbol = 0; secondSymbol < secondInput.Length; secondSymbol++)
                {
                    int secondNumber = int.Parse(secondInput[secondSymbol].ToString());

                    int multiplication = firstNumber * secondNumber + remainder;

                    int numberToAppend = multiplication % 10;
                    remainder = multiplication / 10;

                    currSum.Append(numberToAppend);

                    if (remainder != 0 && secondSymbol + 1 == secondInput.Length)
                    {
                        currSum.Append(remainder);
                    }
                }

                result = SumTwoNumbers(result.ToString(), currSum.ToString().PadLeft((currSum.Length + firstSymbol), '0'));
            }

            if (result.ToString() == string.Empty || result.ToString().All(c => c == '0'))
            {
                Console.WriteLine(0);

                return;
            }

            Console.WriteLine(ReverseStringNumber(result.ToString()));
        }

        public static StringBuilder SumTwoNumbers(string firstNumber, string secondNumber)
        {
            StringBuilder result = new StringBuilder();

            int smallerLen = Math.Min(firstNumber.Length, secondNumber.Length);
            int remainder = 0;
            for (int currNumber = 0; currNumber < smallerLen; currNumber++)
            {
                int sum = int.Parse(firstNumber[currNumber].ToString()) + int.Parse(secondNumber[currNumber].ToString()) + remainder;

                int numberToAppend = sum % 10;
                remainder = sum / 10;

                result.Append(numberToAppend);

                if (remainder != 0 && (currNumber + 1) == smallerLen && (firstNumber.Length == secondNumber.Length))
                {
                    result.Append(remainder);
                }
            }

            if (firstNumber.Length == secondNumber.Length)
            {
                return result;
            }

            for (int currNumber = smallerLen; currNumber < firstNumber.Length; currNumber++)
            {
                int sum = int.Parse(firstNumber[currNumber].ToString()) + remainder;

                int numberToAppend = sum % 10;
                remainder = sum / 10;

                result.Append(numberToAppend);

                if (currNumber + 1 == firstNumber.Length && remainder != 0)
                {
                    result.Append(remainder);
                }
            }

            for (int currNumber = smallerLen; currNumber < secondNumber.Length; currNumber++)
            {
                int sum = int.Parse(secondNumber[currNumber].ToString()) + remainder;

                int numberToAppend = sum % 10;
                remainder = sum / 10;

                result.Append(numberToAppend);

                if (currNumber + 1 == secondNumber.Length && remainder != 0)
                {
                    result.Append(remainder);
                }
            }

            return result;
        }

        public static string ReverseStringNumber(string numberToReverse)
        {
            StringBuilder toReturn = new StringBuilder();
            for (int currSymbol = 0; currSymbol < numberToReverse.Length; currSymbol++)
            {
                toReturn.Append(numberToReverse[numberToReverse.Length - 1 - currSymbol]);
            }

            return toReturn.ToString();
        }
    }
}