#include <iostream>
using namespace std;

struct elem {
    int key;
    elem *next;
} *left = NULL, *right = NULL;

void push_r(int n) {
    elem *p;
    p = right;
    right = new elem;
    right->key = n;
    right->next = NULL;
    if (left == NULL)
        left = right;
    else
        p->next = right;
}

int pop_l(int &n) {
    elem *p;
    if (left) {
        n = left->key;
        p = left;
        left = left->next;
        if (left == NULL)
            right = NULL;
        delete p;
        return 1;
    } else {
        return 0;
    }
}