#include <iostream>
#include <string>
using namespace std;
struct Node
{
    int var;
    Node *Next;
};
typedef Node *PNode;

bool Check_Empty(PNode Head, PNode Tail)
{
    if(Head == NULL && Tail == NULL) return true;
    else return false;
}

void Output(PNode Head)
{
    PNode p = Head;
    if(Head != NULL)
    {
        cout << "Текущий список:" << endl;
        while(p != NULL)
        {
            cout << p -> var << " ";
            p = p -> Next;
        }
    }
    else cout << "Список пуст.";
    cout << endl;
}

void Push(PNode &Head, PNode &Tail, int x)
{
    PNode p;
    p = new Node;
    p -> var = x;
    p -> Next = NULL;
    if(Head != NULL)
    {
        Tail -> Next = p;
        Tail = p;
    }
    else
    {
        Head = p;
        Tail = p;
    }
    cout << "ok" << endl;
}

void Pop(PNode &Head, bool Del)
{
    int t = Head -> var;
    if(Del)
    {
        PNode Old = Head;
        Head = Head -> Next;
        delete Old;
    }
    cout << t << endl;
}

int Size(PNode Head)
{
    PNode p = Head;
    int n = 0;
    while(p != NULL)
    {
        n++;
        p = p -> Next;
    }
    return n;
}

void Clear(PNode &Head)
{
    while(Head != NULL)
    {
        PNode Old = Head;
        Head = Head -> Next;
        delete Old;
    }
    cout << "ok" << endl;
}

void main()
{
    PNode Head = NULL;
    PNode Tail = NULL;
    string operation;
    while(cin >> operation)
    {
        if(operation == "check") Output(Head);
        if(operation == "push")
        {
            int x;
            cin >> x;
            Push(Head, Tail, x);
        }
        if(operation == "pop")
        {
            if(Check_Empty(Head, Tail))
                cout << "error" << endl;
            else Pop(Head, true);
        }
        if(operation == "front")
        {
            if(Check_Empty(Head, Tail))
                cout << "error" << endl;
            else Pop(Head, false);
        }
        if(operation == "size") cout << Size(Head) << endl;
        if(operation == "clear") Clear(Head);
        if(operation == "exit")
        {
            cout << "bye" << endl;
            return;
        }
    }
}