#include<stdio.h>
#include<string.h>

#define max 100

int main(){
    int stack[max];
    int top=-1;
    int q,x;
    char command[10];
    scanf("%d",&q);
    if(q<0){
        printf("Invalid input\n");
        return 0;
    }
    for(int i=0;i<q;i++){
        scanf("%s",command);
        if(strcmp(command,"PUSH")==0){
            scanf("%d",&x);
            if(top<max-1){
                stack[++top]=x;
            }
        }
        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\n");
                else{
                    for(int j=0;j<=top;j++){
                        printf("%d",stack[j]);
                        printf("\n");}
                }
            }
            return 0;
}