#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();
        }
        else{
            printf("Invalid input");
            return 0;
        }
        
    }
    return 0;
}