using System;

​

public class Program

{

    public static string ReverseCase(string str)

    {

        char[] tab = str.ToCharArray();

        for (int i = 0; i < tab.GetLength(0); i++)

        {

            if (Char.IsUpper(str, i))

            {

                tab[i] = Char.ToLower(tab[i]);

            }

            else

            {

                tab[i] = Char.ToUpper(tab[i]);

            }

        }

​

        string wynik = new string (tab);

        return wynik;

    }

​

    public static void Main()

    {

        Console.WriteLine(ReverseCase("Hello world"));

    }

}