#include<stdio.h>
#include<stdlib.h>
int top = -1;
int arr[25];
int n = 0;
int isEmpty(){
    return top == -1;
}
int isFull(){
    return top == n-1;
}

void push(int num){
    // if(isFull()){
    //     printf("Stack overflow");
    // //     return;
    // // }else{
    //     arr[++top] = num;
    // // }
}

void pop(){
    if(isEmpty()){
        printf("Stack Underflow");
        return;
    }else{
        top--;
    }
}

void peek(){
    if(isEmpty()){
        printf("Stack overflow");
        return;
    }else{
        printf("%d",arr[top]);
    }
}

void display(){
    int itr;
    for(itr=0;itr<=top;itr++){
        printf("%d ",arr[itr]);
    }
}
int main(){
    int n,num,choice;
    scanf("%d",&n);
    if(n<0){
        printf("Invalid input");
        return 0;
    }
    push(20);
    display();
    int itr=0;
    while(itr<n){
        scanf("%d",&num);
        printf("%d"num);
        switch(num){
        case 1:
            int input;
            scanf("%d",&input);
            push(input);
            break;
        case 2:
            pop();
            break;
        case 3:
            peek();
            break;
        case 4:
            display;
            break;
        default:
            printf("Invalid input");
    }
    itr++;
    }
   
    
}