#include <cstdlib>
#include <string>
#include <cstdio>


using namespace std;

struct ListNode {
    ListNode *mNext;
    int mData;
};


class Queue {
public:
    // конструктор
    Queue() {
        mHead = nullptr;
    }

    // деструктор
    ~Queue() {
        ListNode *cur = mHead;
        ListNode *before;
        while (cur != nullptr) {
            before = cur->mNext;
            delete cur;
            cur = before;
        }
    }

    void pop() {
        ListNode *dl = mHead;
        while (dl->mNext != nullptr) {
            dl = dl->mNext;
        }
        delete dl->mNext;
        dl->mNext = nullptr;
    }

    void push(const int tag) {
        ListNode *new_el = new ListNode;
        new_el->mData = tag;
        new_el->mNext = mHead;
        mHead = new_el;
    }

    int top() {
        return mHead->mData;
    }

    bool empty() {
        return mHead == nullptr;
    }

    size_t size() {
        ListNode *cur = mHead;
        size_t kl = 0;
        while (cur != nullptr) {
            cur = cur->mNext;
            kl++;
        }
        return kl;
    }


private:

protected:
    ListNode *mHead;
};