Factorial of large numbers

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

 

namespace ConsoleApplication1

{

    class Program

    {

        static void Main(string[] args)

        {

            string s1 = "30";

            int num1 = int.Parse(s1);

            int num2 = num1 - 1;

            //string s2 = num2.ToString();

            string ans = multiply(s1, num2);

            while (num2 > 1)

            {

                num2--;

                //s2 = num2.ToString();

                ans = multiply(ans, num2);

            }

            Console.WriteLine(ans);

        }

        static string multiply(string s1, int s2)

        {

            int length1 = s1.Length;

            int[] arr = new int[length1];

            for (int i = 0; i < length1; i++)

            {

                string str = "";

                str = str + s1[i];

                arr[i] = int.Parse(str);

            }

            int carry = 0;

            string answer = "";

            //Multiplying the second number(full) to the first number(digitwise).

            for (int i = length1-1; i >= 0; i--)

            {

                int ans = arr[i] * s2 + carry;

                int dig = ans % 10;

                carry = ans / 10;

                answer = answer + dig.ToString();

            }

            //Prefixing the remaining carry to the answer.

            if (carry != 0)

            {

                while (carry > 0)

                {

                    int m = carry % 10;

                    answer = answer + m.ToString();

                    carry = carry / 10;

                }

            }

            string Finalanswer = "";

            //Reversing the answer string to get the Finalanswer.

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

            {

                Finalanswer = Finalanswer + answer[i];

            }

            return Finalanswer;

        }

    }

}