using System;
using System.Collections.Generic;
using System.Linq;

public class TrainComposition
{
        List<int> train = new List<int>();

        public void AttachWagonFromLeft(int wagonId)
        {
            if(train != null)
            {
                List<int> newWagon = new List<int>(train.Count + 1);
                newWagon.Add(wagonId);

                foreach (int wagon in train)
                    newWagon.Add(wagon);

                train = newWagon;
            }
            else
                train.Add(wagonId);
        }

        public void AttachWagonFromRight(int wagonId)
        {
            train.Add(wagonId);
        }

        public int DetachWagonFromLeft()
        {
            int wagon = train[0];
            train.RemoveAt(0);

            return wagon;
        }

        public int DetachWagonFromRight()
        {
            int wagon = train[train.Count-1];
            train.RemoveAt(train.Count - 1);

            return wagon;
        }

    public static void Main(string[] args)
    {
        TrainComposition tree = new TrainComposition();
        tree.AttachWagonFromLeft(7);
        tree.AttachWagonFromLeft(13);
        Console.WriteLine(tree.DetachWagonFromRight()); // 7
        Console.WriteLine(tree.DetachWagonFromLeft()); // 13
    }
}