#include <iostream>
#include <cstring>

using namespace std;

typedef struct link
{
    link * next;
    int x;
}*LINK;

string s;
int t;

int main ()
{
    int n = 0;

    LINK head = new link;
    head->next = NULL;

    cin >> n;

    while (n > 0)
    {
        n --;
        cin >> s;
        if (s == "push")
        {
            cin >> t;
            LINK temp = head;
            while (temp->next)
            {
                temp = temp->next;      
            }
            temp->next = new link;
            temp = temp->next;
            temp->x = t;
            temp->next = NULL;
        }
        else if (s == "pop")
        {
            LINK temp = head->next;
            head->next = temp->next;
            delete(temp);
        }
        else if (s == "empty")
        {
            if (head->next == NULL)
            {
                cout <<"YES" << endl;
            }
            else 
            {
                cout << "NO" << endl;
            }
        }
        else if (s == "query")
        {
            cout << head->next->x << endl;
        }
    }

    return 0;
}