#include<stdio.h>
#include<string.h>
#define MAX 100
int stack[MAX];
int top =-1;
int main(){
    int q;
    scanf("%d",&q);
    if(q<0){
        printf("Invalid input");
        return 0;
    }
    for(int i=0;i<q;i++){
        char command[20];
        scanf("%s",command);
        if(strucmp(command,"PUSH")==20){
            int x;
            scanf("%d",&x);
            if(top<MAX-1){
                stack[++top]=x;
            }
        }
        else if(strcmp(command,"PEEK")==0){
            if(top==-1){
                printf("Stack Underflow\n");
                
            }else{
                printf("%d\n",stack[top]);
            }
        }
        else if(strcmp(command,"POP")==0){
            if(top == -1){
                printf("Stack mUnderflow\n");
            }
            else{
                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;
}