#include <iostream>

using namespace std;

const int N = 1e5 + 10;

int stk[N];
int top = 0;

int main() 
{
    int m; 
    cin >> m;

    while (m -- ) 
    {
        string op;
        int x;

        cin >> op;
        if (op == "push")               //进栈
        {
            cin >> x;
            stk[top ++ ] = x;
        } 
        else if (op == "pop")           //出栈
        {
            top -- ;                    //不需要真的出栈，只要栈顶指针减1即可
        } 
        else if (op == "empty")         //判空
        {
            if (!top) printf("YES\n");
            else printf("NO\n");
        } 
        else                            //查询栈顶元素
        {
            printf("%d\n", stk[top - 1]);
        }
    }

    return 0;
}