using System;
using System.Collections.Generic;

public class Test
{
	public static void Main()
	{
		var set = new HashSet<int>();

		Console.WriteLine(set.Add(42) ? "Added" : "Not added");
		Console.WriteLine(set.Count);
		
		// повторим
		Console.WriteLine(set.Add(42) ? "Added" : "Not added");
		Console.WriteLine(set.Count);
	}
}