#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAX 100
int top=-1;
int arr[MAX];
int isfull()
    {
      if(top==MAX-1)
        return 1;
    else
     return 0;
    }
    void push(int num)
    {
        if(isfull())
          printf("Stack is full\n");
        else
          arr[++top]=num;
    }
    int isempty()
    {
        if(top==-1)
           return 1;
        else
           return 0;
    }
    void pop()
    {
        if(isempty())
           printf("Stack is empty\n");
        else
          top--;
    }
    int peek()
    {
        if(isempty())
           printf("Stack underflow");
        else
           return arr[top];
    }
    void display(){
    for(int i=0;i<=top;i++)
    {
        printf("%d",arr[i]);
        
    }
    printf("\n");
    }
    int main()
    {
        int size,i,num;
        char choice[20];
        scanf("%d\n",&size);
        if(size<0)
        {
            printf("Invalid input");
            return 0;
        }
        for(i=0;i<size;i++)
        {
            scanf("%s",choice); 
            if(strcmp(choice,"PUSH")==0){
                scanf("%d",&num);
                push(num);
            }
            else if(strcmp(choice,"POP")==0){
                pop();
            }
        else if(strcmp(choice,"PEEk")==0){
            printf("%d",peek());
        }
        else if(strcmp(choice,"DISPLAY")==0)
            display();
        }
    }
    return 0;
    }