using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        List<string[]> list = new List<string[]>();

        list.Add(new string[] { "A", "B" });

        list.Add(new string[] { "C", "D" });

​

        List<string> data = new List<string>();

        data.AddRange(list.SelectMany(s => s).ToList());

​

​

        Console.WriteLine(data.Count());

    }

}