using System;

using System.Text;

​

namespace MyApp

{

    public class Program

    {

        public static void Main(string[] args)

        {

            string input = "BG31UNCR70001523774937";

            Console.WriteLine($"{nameof(input)}={input}");

​

            string rearranged = input.Substring(4, input.Length - 4) + input.Substring(0, 4);

            Console.WriteLine($"{nameof(rearranged)}={rearranged}");

​

            StringBuilder sb = new StringBuilder();

            foreach (var @char in rearranged)

            {

                int temp;

                if (char.IsLetter(@char))

                    temp = @char - 55;

                else

                    temp = short.Parse(@char.ToString());

​

                sb.Append(temp);

            }

​

            var asciiCoded = sb.ToString();

            Console.WriteLine($"{nameof(asciiCoded)}={asciiCoded}");

​

            short checksum = short.Parse(asciiCoded.Substring(0, 1));

            Console.WriteLine($"step 0; checksum={checksum}");

​

            for (var i = 1; i < asciiCoded.Length; i++)

            {

                Console.WriteLine("---------");

                Console.Write($"step {i}; ");

                Console.Write($"checksum={checksum}; ");

​

                Console.Write($"checksum = checksum*10 = {checksum}*{10} = ");

                checksum *= 10;

                Console.Write(checksum);

​

                Console.Write($"; checksum = checksum+asciiCoded[{i}] = {checksum}+{asciiCoded.Substring(i, 1)} = ");

                checksum += short.Parse(asciiCoded.Substring(i, 1));

                Console.Write(checksum);

​

                Console.Write($"; checksum = checksum%97 = {checksum}%97 = ");

                checksum %= 97;

                Console.WriteLine(checksum);

            }

​

            Console.WriteLine($"Is IBAN? {checksum == 1}");

        }

    }

}

​

​