#include <iostream>
#include <string>
#include <vector>
using namespace std;

struct Stack {
	vector<int> A;
	int k = 0;
	
	void push(int n) {
		A.push_back(n);
		k = k + 1;
	}
	
	int back() const {
		return A[k - 1];
	}
	
	int pop() {
		int t = back();
		k = k - 1;
		A.pop_back();
		return t;
	}
	
	int size() const {
		return k;
	}
	
	void clear() {
		k = 0;
		A.clear();
	}
};

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.back() << '\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;
}