#include <stdio.h>
#include <stdlib.h>

typedef struct Node{
    int data;
    struct Node* next;
}Node;

struct Node* top = NULL;

Node* alloc_node(int e){
    Node* p = (Node*)malloc(sizeof(Node));
    p->data = e;
    p->next = NULL;
    return p;
}

int free_node(Node* p){
    int result = p->data;
    free(p);
    return result;
}

int is_empty(){
    if(top == NULL){
        return 1;
    }
    else{
        return 0;
    }
}

void push(int e){
    Node* p = alloc_node(e);
    
    if(is_empty()){
        top = p;
    }
    else{
        p->next = top;
        top = p;
    }
}

int pop(){
    if(is_empty()){
        printf("underflow\n");
        exit(0);
    }
    else{
        Node* p = top;
        top = top->next;
        return free_node(p);
    }
}

int print_stack(Node* t){
    if(is_empty()){
        printf("empty\n");
        exit(0);
    }
    else{
        while(t != NULL){
            printf("%d ", t->data);
            t = t->next;
        }
        printf("\n");
    }
}

int main(void){

    int n;

    while(1){
        scanf("%d", &n);

        if(n == 0){
            print_stack(top);
            break;
        }
        else if(n == -1){
            pop();
        }
        else if(n > 0){
            push(n);
        }
    }

    return 0;
}