using System;

public class Program

{

    public static void Main()

    {

               Int32 a,b,c;

       String textik;

       do

        {

           Console.WriteLine("Zadej stranu a : ");

           textik = Console.ReadLine();

           a = Convert.ToInt32(textik);

           if (a==0)

               Console.WriteLine("Stana 'a' se nesmí rovnat nule...");

        } while(a==0);

    }

}