05. Multiply Big Number

using System;
using System.Collections.Generic;
using System.Linq;

namespace 05. Multiply Big Number
{
    class Program
    {
        static void Main(string[] args)
        {
            string bigNumber = Console.ReadLine();
            int singleDigit = int.Parse(Console.ReadLine());
            string result = string.Empty;
            int number = 0;
            int temp = 0;
            int firstDigit = 0;
            int lastDigit = 0;
            int naum = 0;
            if (singleDigit == 0)
            {
                Console.WriteLine(0);
                return;
            }
            for (int i = bigNumber.Length - 1; i >= 0; i--)
            {
                number = int.Parse(bigNumber[i].ToString());
                temp = number * singleDigit;
                lastDigit = temp % 10;
                if (temp >= 10)
                {
                    firstDigit = temp / 10;
                }
                else
                {
                    firstDigit = 0;
                }
                if (i == bigNumber.Length - 1)
                {
                    result += lastDigit;
                }
                else
                {
                    int tempResult = naum + lastDigit;

                    if (tempResult >= 10)
                    {
                        lastDigit = tempResult % 10;
                        result += lastDigit;
                        firstDigit += tempResult / 10;
                    }
                    else
                    {
                        result += tempResult;
                    }

                }
                naum = firstDigit;
            }
            if (firstDigit != 0)
            {
                result += naum;
            }
            string finalString = Reverse(result);
            int lenght = finalString.Length;
            for (int i = 0; i < lenght; i++)
            {
                char ch = finalString[0];
                if(ch == '0')
                {
                    finalString =finalString.Remove(0, 1);
                }
                else
                {

                    break;
                }
            }
            Console.WriteLine(finalString);
        }
        public static string Reverse(string s)
        {
            char[] charArray = s.ToCharArray();
            Array.Reverse(charArray);
            return new string(charArray);
        }
    }
}