#include<stdio.h>
#include<string.h>
#define MAX 100
int main()
{
    int q;
    scanf("%d",&q);
    if(q<0)
    {
        printf("Invalid input");
        return 0;
    }
    int stack[MAX];
    int top=-1;
    char command[10];
    int value;
    for(int i=0;i<q;i++)
    {
        scanf("%s",command);
        if(strcmp(command,"PUSH")==0)
        {
            scanf("%d",&value);
            if(top<MAX-1)
            {
                stack[++top]=value;
            }
        }
        else if(strcmp(command,"POP")==0)
        {
            if(top==-1)
            {
            printf("stack underflow\n");
        }
        else
        {
            top--;
        }
    }
    else if(strcmp(command,"PEEK")==0)
    {
        if(top==-1)
        {
            printf("stack underflow\n");
        }
        else
        {
            printf("%d\n",stack[top]);
        }
    }
    else if(strcmp(command,"DISPLAY")==0)
    {
        if(top==-1)
        {
            printf("stack underflow");
            
        }
    }
    else
    {
        for(int j=0;j<=top;j++)
        {
            printf("%d",stack[j]);
        }
        printf("\n");
    }
    return 0;
    
}