#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
using namespace std;
const int N = 100010;
int idx=1,head, e[N], ne[N];

//初始化
void init()
{
    head = -1;
    idx = 1;
}
//在头结点增加节点
void add_head(int x)
{
    e[idx] = x, ne[idx] = head, head = idx ++ ;
}
//压栈
void push(int u)
{
    add_head(u);    
}
//弹栈
void pop()
{
    head = ne[head];
}
//判断是否为空
bool empty()
{
    if(head == -1)  return true;
    else return false;
}
//获取栈顶元素
int ftop()
{
    return e[head];
}

int main()
{
    init();
    int n;
    cin >> n;
    while(n--)
    {
        string s;
        cin >> s;

        if(s == "push")
        {
            int a;
            cin >> a;
            push(a);
        }

        if(s == "pop")
        {
            pop();
        }

         if(s == "query")
        {
            cout << ftop() << endl;
        }

        if(s == "empty")
        {
            if(empty()) cout<<"YES"<<endl;
            else cout<<"NO"<<endl;
        }
    }
}