using System;

using System.Collections.Generic;

using System.Collections.Frozen;

public class Program

{

    public static void Main()

    {

        FrozenSet<string> kaupunkilista = new HashSet<string>() {"Lahti", "Helsinki", "Mikkeli"}.ToFrozenSet(optimizeForReading: false);

        Console.WriteLine($"Mikkeli on kaupunki {kaupunkilista.Contains("Mikkeli")}");

    }

}