#include <iostream>
#include <cstring>

using namespace std;

struct Node{
    Node* minKey;
    Node* next;
    int key;
};

bool empty(Node* top){
    if (top == NULL){
        return true;
    }
    return false;
}

int size(Node* top){
    int res = 0;
    while(top != NULL){
        ++res;
        top = top -> next;
    }
    return res;
}

bool isLess(Node* x, Node* y){
    if ((x -> key) <= (y -> key)){
        return true;
    }
    return false;
}

void pop(Node* &top){
    if (empty(top)){
        cout << "error" << '\n';
        return ;
    }
    else {
        cout << (top -> key) << '\n';
        Node* temp = top;
        top = top -> next;
        delete temp;
        return;
    }
}

void push(Node *&top, int key){
    Node* newNode = new Node;
    newNode -> key = key;
    newNode -> next = top;
    if (top == NULL){
        newNode -> minKey = newNode;
    }
    else if (isLess(newNode, top -> minKey)){
        newNode -> minKey = newNode;
    }
    else {
        newNode -> minKey = top -> minKey;
    }
    top = newNode;
    cout << "ok" << '\n';
}

void back(Node* top){
    if (empty(top)){
        cout << "error" << '\n';
        return;
    }
    cout << (top -> key) << '\n';
}

void clear(Node* &top){
    Node* temp;
    while(top != NULL){
        temp = top;
        top = top -> next;
        delete temp;
    }
    cout << "ok" << '\n';
}

void min(Node* top){
    if (empty(top)){
        cout << "error" << '\n';
        return ;
    }
    cout << (top -> minKey) -> key << '\n';
}

int main(){
    int n;
    cin >> n;
    Node* top;
    top = NULL;
    char temp[10];
    for (int i = 0; i < n; ++i){
        char command[10] = "";
        cin >> temp;
        strcat(command, temp);
        if (!strcmp(command, "push")){
            int key;
            cin >> key;
            push(top, key);
        }
        else if(!strcmp(command, "pop")){
            pop(top);
        }
        else if(!strcmp(command, "size")) {
            cout << size(top) << '\n';
        }
        else if(!strcmp(command, "back")) {
            back(top);
        }
        else if(!strcmp(command, "min")) {
            min(top);
        }
        else if(!strcmp(command, "clear")){
            clear(top);
        }
    }
}