v4, C# - rextester

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
         string left ="";
        string right ="";
        string temp ="";
           int num;
        string str5 = "";
        string str6 = "";
        string st1 = "";
        string st2 = "";
        string st3 = "";
        int num2 = 0;
  
       string str = "ABCDEFGHIJKLMNOPQRSTUVWXY"; //MAIN INPUT STRING WE PROVIDE

string str2 = str.Substring(0, 3) + str.Substring(5, 2) + str.Substring(8, 1) + str.Substring(13, 3) + str.Substring(0x12, 1);
    
    string str3 = str.Substring(3, 2) + str.Substring(7, 1) + str.Substring(9, 4) + str.Substring(0x10, 2) + str.Substring(0x13, 1);
         
    string str4 = str.Substring(str.Length - 5, 5);
        
      
        
    if (str4.Substring(0, 1) != str2.Substring(4, 1))
    {
        
        // if program comes here then Input string is not valid
        Console.WriteLine("input string is not valid");
    }
    if (str4.Substring(1, 1) != str3.Substring(0, 1))
    {
           // if program comes here then Input string is not valid
         Console.WriteLine("input string is not valid");
        
    }
    if (str4.Substring(str4.Length - 1, 1) != str2.Substring(0, 1))
    {
        
          // if program comes here then Input string is not valid
         Console.WriteLine("input string is not valid");
        
    }
       
            
    for (num = 0; num < str3.Length; num++)
    {
        
        
        str5 = str5 + Convert.ToString((int) (Convert.ToChar(str3.Substring(num, 1)) - '0')).Substring(Convert.ToString((int) (Convert.ToChar(str3.Substring(num, 1)) - '0')).Length - 1, 1);
        
    }
        
    for (num = 0; num < str2.Length; num++)
    {
        
        str6 = str6 + Convert.ToString((int) Convert.ToChar(str2.Substring(num, 1))).Substring(Convert.ToString((int) Convert.ToChar(str2.Substring(num, 1))).Length - 1, 1);
    
    }
     
        
    if (str5 != str6)
    {
          // if program comes here then Input string is not valid
         Console.WriteLine("input string is not valid");
    }
   
    int num3 = 0;
    string str12 = "";
    for (num = 0; num < str3.Length; num++)
    {
        num3 = Convert.ToChar(str3.Substring(num, 1));
        str12 = Convert.ToString((int) (Convert.ToChar(str3.Substring(num, 1)) - '0')).Substring(Convert.ToString((int) (Convert.ToChar(str3.Substring(num, 1)) - '0')).Length - 1, 1);
        num2 += Convert.ToInt32(str12);
       
    }
   
    str5 = num2.ToString();
    str4 = str4.Substring(2, 2);
    str4 = Convert.ToString((int) (Convert.ToChar(str4.Substring(0, 1)) - '0')).Substring(Convert.ToString((int) (Convert.ToChar(str4.Substring(0, 1)) - '0')).Length - 1, 1) + Convert.ToString((int) (Convert.ToChar(str4.Substring(1, 1)) - '0')).Substring(Convert.ToString((int) (Convert.ToChar(str4.Substring(1, 1)) - '0')).Length - 1, 1);
    if (str5 != str4)
    {
           // if program comes here then Input string is not valid
         Console.WriteLine("input string is not valid");
        
    }
         //final place.   
        //If can come here without any input string error, then input string is valid

}
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,13 sec, cpu time: 0,11 sec, average memory usage: 15 Mb, average nr of threads: 3