#include <iostream>
#include <fstream>
#include <string>

using namespace std;

struct el
{
    int val;
    el* link;
};

int main()
{
    int st_size=0;
    el head;
    head.link=NULL;
    string order="";
    fstream fin;
    fin.open("input.txt");
    while (order!="exit")
    {
        if (order=="push")
            {
                int n;
                fin >>n;
                el* y = new el;
                (*y).link=&head;
                (*y).val=n;
                head=*y;
                st_size++;
                cout <<"ok" <<endl;
            }
        else if (order=="pop")
            {
                if (st_size==0) cout <<"error" <<endl;
                else {
                        cout <<head.val <<endl;
                        el* y = &head;
                        head=*(head.link);
                        delete y;
                        st_size--;
                }
            }
        else if (order=="back")
            {
                if (st_size==0) cout <<"error" <<endl;
                else cout <<head.val <<endl;
            }
        else if (order=="size")
            {
                cout <<st_size << endl;
            }
        else if (order=="clear")
            {
                while (head.link!=NULL)
                {
                    el* y = &head;
                    head=*(head.link);
                    delete y;
                    st_size--;
                }
                cout <<"ok" <<endl;
            }
        fin >>order;
    }
    cout <<"bye" <<endl;
    fin.close();
    return 0;
}