using System;

public class Program

{

    public static void Main()

    {

    int n;

    do{

        Console.WriteLine(" Please selet menu");

        Console.WriteLine(" 1. class A");

        Console.WriteLine(" 2. class B");       

        Console.WriteLine(" 3. class C");

        Console.WriteLine(" 4. class D");

        n = int.Parse(Console.ReadLine());

    }while (n<0 );  

        Console.WriteLine("Welcome to Class.");

    }   

}