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

int main() {
	int A[100];
	int k = 0;
	string s;
	cin >> s;
	
	while (s != "exit") {
		
		if (s == "push") {
			int n;
			cin >> n;
			A[k] = n;
			k = k + 1;
			cout << "ok\n";
		} else if (s == "pop") {
			k = k - 1;
			cout << A[k] << '\n';
		} else if (s == "back") {
			cout << A[k - 1] << '\n';
		} else if (s == "size") {
			cout << k << '\n';
		} else if (s == "clear") {
			k = 0;
			cout << "ok\n";
		}
		
		cin >> s;
	}
	
	cout << "bye\n";
	
	return 0;
}