#include <iostream>
using namespace std;

struct Element {
	int value;
	Element* prev = 0;
};

struct Stack {
	Element* front = 0;
	Element* back = 0;
	int stack_size = 0;
	
	void push(int n) {
		Element* new_elem = new Element();
		new_elem->value = n;
		if (stack_size > 0) {
			new_elem->prev = back;
		} else {
			front = new_elem;
		}
		back = new_elem;
		stack_size++;
	}
	
	int pop() {
		int result = back->value;
		Element* old_back = back;
		if (stack_size > 1) {
			back = back->prev;
		} else {
			front = 0;
			back = 0;
		}
		delete old_back;
		stack_size--;
		return result;
	}
	
	int top() {
		return back->value;
	}
	
	int size() {
		return stack_size;
	}
	
	void clear() {
		while (size() > 0) {
			pop();
		}
	}
};

int main() {
	Stack st;
	
	string s;
	cin >> s;
	
	while (s != "exit") {
		
		if (s == "push") {
			int n;
			cin >> n;
			st.push(n);
			cout << "ok\n";
		} else if (s == "pop") {
			cout << st.pop() << '\n';
		} else if (s == "back") {
			cout << st.top() << '\n';
		} else if (s == "size") {
			cout << st.size() << '\n';
		} else if (s == "clear") {
			st.clear();
			cout << "ok\n";
		}
		
		cin >> s;
	}
	
	cout << "bye\n";
	return 0;
}