using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Tokenize("1234567890123456"));

        Console.WriteLine(Tokenize("123456789012345"));

        Console.WriteLine(Tokenize("12345678901234"));

        Console.WriteLine(Tokenize("1234567890123"));

        Console.WriteLine(Tokenize("123456789012"));

        Console.WriteLine(Tokenize("12345678901"));

        Console.WriteLine(Tokenize("1234567890"));

        Console.WriteLine(Tokenize("123456789"));

        Console.WriteLine(Tokenize("12345678"));

        Console.WriteLine(Tokenize("1234567"));

        Console.WriteLine(Tokenize("123456"));

        Console.WriteLine(Tokenize("12345"));

        Console.WriteLine(Tokenize("1234"));

        Console.WriteLine(Tokenize("123"));

    }

    private static string Tokenize(string accountNumber)

    {       

        int bodyLength = accountNumber.Length - 4;

        if (bodyLength <= 0)

            throw new Exception($"accountNumber length of {accountNumber.Length} is too short.");

        int headerLength = bodyLength / 2 + bodyLength % 2;

        string header = accountNumber.Substring(0, headerLength);

        Console.Write(header + "-");

        string middle = accountNumber.Substring(headerLength, bodyLength / 2);

        Console.Write(middle + "-");

        string tail = accountNumber.Substring(bodyLength);

        Console.Write(tail + Environment.NewLine);

        return header + middle + tail;

    }

}