#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 cmd[10];
    int value;
    for(int i=0;i<q;i++)
    {
        scanf("%s",cmd);
        if(strcmp(cmd,"PUSH")==0){
            scanf("%d",&value);
            if(top<MAX-1){
                stack[++top]=value;
            }else if(strcmp(cmd,"POP")==0){
                if(top==-1){
                    printf("Stack underflow");
                }else{
                    top--;
                }
            }else if(strcmp(cmd,"PEEK")==0){
                if(top==-1){
                    printf("Stack underflow\n");
                }else{
                    printf("%d\n",stack[top]);
                }
            }else if(strcmp(cmd,"DISPLAY")==0){
                if(top==-1){
                    printf("Stack underflow");
                }else{
                    for(int i=0;i<=top;i++)
                    printf("%d",stack[i]);
                }
                printf("\n");
            }
        }
    }
    return 0;
}