using System;

using System.Collections.Generic;

​

namespace ConsoleApp1

{

    class Program

    {

        static void Main(string[] args)

        {

            var s = "";

            var st = new MyStack();

            while (true)

            {

                s = Console.ReadLine();

                if (s == "exit")

                {

                    break;

                }

                else

                if (s.StartsWith("push"))

                {

                    var n = int.Parse(s.Split(' ')[1]); // [push, 10]

                    st.Push(n);

                    Console.WriteLine("ok");

​

                }

                else if (s == "pop")

                {

​

                    Console.WriteLine(st.Pop());

                }

                else if (s == "back")

                {

​

                    Console.WriteLine(st.Back());

                }

                else if (s == "size")

                {

                    Console.WriteLine(st.Size());

                }

                else if (s == "clear")

                {

                    st.Clear();

                    Console.WriteLine("ok");

                }

                // 

                s = "";

            }

            Console.WriteLine("bye");

        }

    }

}

public class MyStack

{

    List<int> stack = new List<int>();

    public MyStack()

    {

​

    }

    public void Push(int value)

    {

        stack.Add(value);

​

    }

    public int Pop()

    {

        var value = stack[stack.Count - 1];

        stack.RemoveAt(stack.Count - 1);

        return value;

    }

    public int Back()

    {

        return stack[stack.Count - 1];

​

    }

    public int Size()

    {

        return stack.Count;

    }

    public void Clear()

    {

        stack.Clear();

​

    }

}

​