using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var elNum = new Stack<int>();

        elNum.Push(100);

        elNum.Push(200);

        elNum.Push(300);

        elNum.Push(400);

        Console.Write("Number of elements in Stack: {0}", elNum.Count); // 4

        Console.WriteLine();

        if (elNum.Count > 0)  // (A)

        {

            Console.WriteLine(elNum.Peek()); // 400

            Console.WriteLine(elNum.Peek()); // 400

        }

        Console.WriteLine("At the end the number of elements is: {0}",

                                                                elNum.Count);

        elNum.Clear();  // clears all elements from the stack  (B)

        //elNum.Peek(); // Run-time exception: Stack empty     (C)

    }

}