import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a[] = new int[100010] , tt = -1;
        int q = sc.nextInt();
        while(q-- != 0){
            String s = sc.next();
            if("push".equals(s))  a[++tt] = sc.nextInt();
            else if("pop".equals(s)) tt --;
            else if("query".equals(s)) System.out.println(a[tt]);
            else System.out.println(tt == -1 ? "YES":"NO");
        }
    }

}