#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int size=5,top=-1;
int arr[101];
int isEmpty(){
    if (top==-1){
        return -1;
    }
    else{
        return 0;
    }
}
int isFull(){
    if(top==size-1){
        return -1;
    }else{
        return 0;
    }
}
int peek(){
    if(isEmpty()){
        return -1;
    }else{
        return arr[top];
    }
}
void push(int num){
    if(isEmpty()){
        printf("stack underflow");
    }else{
        arr[++top] = num;
    }
}
void pop(){
    if(isEmpty()){
        printf("stack underflow");
    }else{
        arr[top--1];
    }
}
void traversal(){
    for(int i=0;i<=top;i++){
        printf("%d ",arr[i]);
    }
    printf("\n");
}
 int main(){
     char choice[101];
     int num,n,i;
     scanf("%d",&n);
     if(n<0){
         printf("Invalid input");
         return 0;
     }
     for(intr n-0;i<n;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,"DISPLAY" )==0){
             traversal()
         }else if(strcmp(choice,"PEEK" )==0){
             printf("%d",peek());
         }else{
             printf("Invalid input");
         }
     }
 }