public class StackItem<T>
{
	public T Value;
	public StackItem<T> Previous;
}

public class Stack<T>
{
	public StackItem<T> Tail;
	private int Count;

	public void Push(T item)
	{
		var stackItem = new StackItem<T> { Value = item };
		if (Count == 0)
		{
			stackItem.Previous = null;
			Tail = stackItem;
			Count++;
			return;
		}
		stackItem.Previous = Tail;
		Tail = stackItem;
		Count++;
	}

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