template <class T>
class List {
protected:
    struct Node {
        T value;
        T* next;
        Node(T& v = T()) : value(v), next(0) {}
    };
    Node* head;
    Node* tail;
public:
    List();
    List(const List&);
    List& operator =(const List&);
    ~List();
    void add(const T&);
    void remove(Node*);
};
template <class Tp>
class My_stack : public List<Tp> {
public:
    void push(const Tp& v) { add(v); }
    void pop()             { remove(this->tail); }
    Tp& top() const        { return *(this->tail); }
    bool empty() const     { return !(this->head) ; }
    size_t size() const
    {
        if (empty())  return 0;
        size_t sz{};
        for (typename List<Tp>::Node* p = this->head; p; p = p->next)
            ++sz;
    }
};