using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TodoApplication
{
    public class ESP<T>
    {
        public T Value { get; set; }
        public ESP<T> Next { get; set; }
        public ESP<T> Previous { get; set; }
    }

    public class LimitedSizeStack<T>
    {
        private ESP<T> Head;
        private ESP<T> Tail;
        private readonly int length;

        public LimitedSizeStack(int limit)
        {
            length = limit;
        }

        public void Push(T item)
        {
            ESP<T> Item = new ESP<T> { Value = item };

            if (Head == null) {
                Head = Tail = Item;
            } else {
                if (length == Count)
                {
                    Head = Head.Next;
                    --Count;
                }
                    Item.Previous = Tail;
                    if (Tail != null)
                    {
                        Tail.Next = Item;
                    }
            }
            Tail = Item;
            ++Count;
        }

        public T Pop()
        {
            if (Count == 0) throw new InvalidOperationException();
            var res = Tail.Value;
            Tail = Tail.Previous;
            --Count;
            return res;
        }

        public int Count { get; private set; }
    }
}