View difference between Paste ID: irQBAkzj and SESFMbub

 using System;
 using System.Text;
 
 class CalculateNFacturielwithDigitsInArray
 {
     static void Main()
     {
         Console.WriteLine("Insert Number to calculate its facturial:");
         string n = Console.ReadLine();
         //n = MultiplyTwoNumbersByDigits("85", "135");
         string facturiel = CalcFacturiel(n);
         Console.WriteLine(facturiel);
     }
 
     private static string CalcFacturiel(string n)
     {
         if (n.Length == 1)
         {
             if (Convert.ToInt32(n) > 1)
             {
                 n = MultiplyTwoNumbersByDigits(n, CalcFacturiel(numberMinusOne(n)));
             }
         }
         else
         {
             n = MultiplyTwoNumbersByDigits(n, CalcFacturiel(numberMinusOne(n)));
         }
         return n;
     }
     static string numberMinusOne(string n)
     {
         StringBuilder sb = new StringBuilder();
         sb.Append(n);
         int number;
         int i = 1;
         do
         {
             number = Convert.ToInt32(sb[sb.Length - i].ToString());
 
             if (number != 0)
             {
                 sb.Remove(sb.Length - i, 1);
-                if (number - 1 != 0 && sb.Length == i)
+                if (number - 1 != 0 || sb.Length >= i)
                 {
                   sb.Insert(sb.Length - i + 1, (number - 1));
                 }
             }
             i++;
         }
         while (number == 0);
         i--;
         sb.Replace("0", "9", sb.Length + 1 - i, i -1);
 
         n = sb.ToString();
         return n;
     }
 
     static string MultiplyTwoNumbersByDigits(string firstNumber, string secondNumber)
     {
         int[] firstArray = new int[firstNumber.Length];
         int[] secondArray = new int[secondNumber.Length];
         int[] multiArray = new int[Math.Max(firstNumber.Length, secondNumber.Length) * 2];
         int[] multiArrayCurrent = new int[multiArray.Length];
         int[] multiArrayPrevious = new int[multiArray.Length];
         StringBuilder Number = new StringBuilder();
         Number.Append(firstNumber);
         for (int i = 0; i < firstNumber.Length; i++)
         {
             firstArray[i] = Convert.ToInt32(Number[firstNumber.Length - 1 - i].ToString());
         }
         Number.Clear();
         Number.Append(secondNumber);
         for (int i = 0; i < secondNumber.Length; i++)
         {
             secondArray[i] = Convert.ToInt32(Number[secondNumber.Length - 1 - i].ToString());
         }
         Number.Clear();
         for (int j = 0; j < firstNumber.Length; j++)
         {
             for (int i = 0; i <secondNumber.Length + 1; i++)
             {
                 if (i == 0)
                 {
                     multiArrayCurrent[i + j] = (firstArray[j] * secondArray[i]) % 10;
                 }
                 else if (i == Math.Max(firstNumber.Length, secondNumber.Length))
                 {
                     multiArrayCurrent[i + j] = (firstArray[j] * secondArray[i - 1]) / 10;
                 }
                 else
                 {
                     multiArrayCurrent[i + j] = (firstArray[j] * secondArray[i]) % 10 + (firstArray[j] * secondArray[i - 1]) / 10;
                 }
             }
             int naum = 0;
             for (int i = 0; i < multiArray.Length; i++)
             {
                 if (i == 0)
                 {
                     multiArray[i] = (multiArrayCurrent[i] + multiArrayPrevious[i]) % 10;
                 }
                 else if (i == multiArray.Length)
                 {
                     multiArray[i] = (multiArrayCurrent[i - 1] + multiArrayPrevious[i - 1]) / 10;
                 }
                 else
                 {
                     multiArray[i] = (multiArrayCurrent[i] + multiArrayPrevious[i]) % 10 + (multiArrayCurrent[i - 1] + multiArrayPrevious[i - 1]) / 10 +naum;
                     naum = 0;
                     if (multiArray[i] > 9)
                     {
                         naum = multiArray[i] / 10;
                         multiArray[i] = multiArray[i] % 10;
                     }
                     }
             }
 
             multiArray.CopyTo(multiArrayPrevious, 0);
             Array.Clear(multiArrayCurrent,0,multiArrayCurrent.Length);
 
 
         }
         Array.Reverse(multiArray);
         if (multiArray[0] != 0)
         {
             Number.Append(multiArray[0]);
         }
         bool isZero = false;
         for (int i = 1; i < multiArray.Length; i++)
         {
             if (multiArray[i] != 0 || isZero)
             {
                 Number.Append(multiArray[i]);
                 isZero = true;
             }
         }
         return Number.ToString();
     }
 }