#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;
/*
只用考虑怎么移动栈顶指针就行了
*/
const int N = 100010;

int m;
int s[N];
int h;

int main()
{
    cin >> m;
    while (m -- )
    {
        string op;
        int x;
        cin >> op;
        if(op == "push")
        {
            cin >> x;
            s[h ++ ] = x;
        }
        else if(op == "pop")
        {
            h -- ;
        }
        else if(op == "query")
        {
            cout << s[h - 1] << endl;
        }
        else
        {
            if(h == 0)
                puts("YES");
            else
                puts("NO");
        }
    }
    return 0;
}