using System;

using System.Collections;

public class Program

{

    public static void Main()

    {

            Stack stack = new Stack();

            bool f = true;

            while (true)

            {

                string command = Console.ReadLine();

                if (command == "exit") break;

                if (command == "url")

                {

                    stack.Push(Console.ReadLine());

                }

                if (command == "back")

                {

                    if (f == true) stack.Pop();

                    Console.WriteLine(stack.Pop());

​

                    f = false;

                }

                command = Console.ReadLine();

            }

    }

}