#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){
                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");
                }
                else
            {
            
                printf("%d\n",stack[top]);
                
                
            }
            }
            else if(strcmp(command,"DISPLAY")==0){
                if(top==-1){
                    printf("stack underflow\n");
                }
                else{
                    for(int j=0;j<=top;j++){
                        printf("%d",stack[j]);
                    }
                }
                printf("\n");
            }
        }
    }
    return 0;
}