#include <iostream>

const int cst = -100;

struct Element {
    int value, size = 0;
    Element *prev = nullptr; // prev указывает на element
};

struct Stack {
    Element *p = new Element();

    void push(int n) {
        Element *s = new Element();
        s->value = n;
        s->prev = p;
        s->size = p->size + 1;
        p = s;
        std::cout << "ok\n";
    }

    int back() {
        if (p->size == 0) {
            return cst;
        }
        return p->value;
    }

    int pop() {
        if (p->size == 0) {
            return cst;
        }
        Element *s = p;
        int x = p->value;
        p = p->prev;
        delete s;
        return x;
    }

    void size() {
        std:: cout << p->size <<"\n";
    }

    void clear() {
        delete p;
        p = new Element();
        std::cout << "ok\n";
    }

};

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);


    Stack *n1 = new Stack();
    std::string l;
    std::cin >> l;
    while(l != "exit") {

        if(l == "back") {
            int x = n1->back();
            if(x != cst) {
                std::cout << x << "\n";
            }
            else {
                std::cout << "error\n";
            }
        }

        if(l == "push") {
            int x;
            std::cin >> x;
            n1->push(x);
        }
        if(l == "pop") {
            int x = n1->pop();
            if(x != cst) {
                std::cout << x << "\n";
            }
            else {
                std::cout << "error\n";
            }
        }
        if(l == "size") {
            n1->size();
        }
        if(l == "clear") {
            n1->clear();
        }
        std::cin >> l;
    }
    std::cout << "bye\n";
    delete n1;
}