#include <iostream>
#include <stack>

using namespace std;

void workWithStack (){
    stack<char> simpleStack(100);
    string justInput;
    cin >> justInput;
    char num;
    while (justInput != "exit") {
        if (justInput == "push") {
            cin >> num;
            simpleStack.push(num);
            cout << "ok" << endl;
        } else if (justInput == "pop") {
            simpleStack.peek(1);
            simpleStack.pop();
        } else if (justInput == "back") {
            simpleStack.peek(1);
        } else if (justInput == "size") {
            simpleStack.size();
        } else if (justInput == "clear") {
            simpleStack.clear();
            cout << "ok" << endl;
        }
        cin >> justInput;
    }
    cout << "bye";
}

int main()
{
    workWithStack();
    return 0;
}