#include<stdio.h>
#define MAX 100

int stack[MAX];
int top = -1;

void push(int q)
{
    if(top==MAX-1)
    {
        printf("Stack Underflow\n");
    }
    else
    {
        top++;
        stack[top]=q;
        printf("%d",q);
    }
}
void pop()
{
    if(top==-1)
    {
        printf("Stack underflow\n");
    }
    else
    {
        printf("%d",stack[top]);
        top--;
    }
}
void peek()
{
    if(top==-1)
    {
        printf("Stack is empty");
    }
    else
    {
        printf("%d",stack[top]);
    }
}
void display()
{
    if(top==-1)
    {
        printf("Stack is empty");
    }
    else
    {
        for(int i=top;i>=0;i--)
        {
            printf("%d",stack[i]);
        }
    }
}

int main()
{
    int q,value;
    scanf("%d",&q);
    
    switch(q)
    {
        case 1:
        scanf("%d",&value);
        push(value);
        break;
        case 2:
        pop();
        break;
        case 3:
        peek();
        break;
        case 4:
        display();
        break;
        default:
        printf("Invalid input");
    }
    while(value!=5);
    return 0;
}