#include <stdio.h>
#include <iostream>
using namespace std;
struct Queue {
    int data;
    Queue *prev;
    Queue(int x) {
        data = x;
        prev = NULL;
    }
};
Queue *first = NULL;
Queue *last = NULL;
void push(int x) {
    Queue *t = new Queue(x);
    if (first == NULL) {
        first = t;
        last = t;
        return;
    }
    last->prev = t;
    last = t;
}
int pop() {
    if (first == NULL) return 0;
    int res = first->data;
    first = first->prev;
    return res;
}
int front() {
    if (first != NULL) return first->data;
    return 0;
}
int res = 0;
int size(Queue *t) {
    if (t != NULL) {
        res++;
        size(t->prev);
    }
    if (t == NULL) return 0;
    return res;
}
void clear(Queue *t) {
    if (t != NULL) {
        clear(t->prev);
        pop();

    }
}
int compare(char *str1, char *str2) {
    int i = 0;
    while (str1[i] != '\0' || str2[i] != '\0') {
        if (str1[i] == str2[i]) {
            i++;
        }
        else {
            return 0;
        }
    }
    return 1;

}
int main() {
    char command[10] = { 0 };
    int i = 0;
    int value;
    while (compare(command, "exit") != 1) {
        cin >> command;
        if (compare(command, "push") == 1) {
            cin >> value;
            push(value);
            cout << "ok" << "\n";
        }
        if (compare(command, "pop") == 1) {
            if (pop() == 0) cout << "error"<<"\n";
            else cout << pop() << "\n";
        }
        if (compare(command, "front") == 1) {
            if (front() == 0) cout << "error" << "\n";
            else cout << front() << "\n";
        }
        if (compare(command, "size") == 1) {
            cout << size(first) << "\n";
            res = 1;
        }
        if (compare(command, "exit") == 1) {
            cout << "bye";
        }
        if (compare(command, "clear") == 1) {
            clear(first);
            cout << "ok" << "\n";
        }
    }
    return 0;
}