using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        // store the list as is - this is already in your code

        var myList = new List<string>();

        myList.Add("A1");

        myList.Add("A2");

        myList.Add("A3");

        myList.Add("A4");

        // store a copy of the list in ascending order

        var ascList = myList.OrderBy(a => a);

        // store the first letter in a list.

        // if the distict count results in more than one, then it should be an error

        var codeBeginsWith = new List<char>();

        myList.ForEach(a => codeBeginsWith.Add(a.First()));

        codeBeginsWith = codeBeginsWith.Distinct().ToList();        

        // 1. Check to see if it's in sequential order

        // 2. Check to make sure no other code is used

        if (!myList.SequenceEqual(ascList) || codeBeginsWith.Count > 1) {

          Console.WriteLine("Not in Sequential Order"); 

        }

        Console.WriteLine("Complete");

    }

}