using System.Threading.Tasks;
public static async Task Main(string[] args)
Console.Write("Введите адрес сайта: ");
string url = Console.ReadLine();
var httpClient = new HttpClient();
var html = await httpClient.GetStringAsync(url);
var htmlDocument = new HtmlDocument();
htmlDocument.LoadHtml(html);
var links = htmlDocument.DocumentNode.Descendants("a")
.Select(a => a.GetAttributeValue("href", null))
.Where(href => !String.IsNullOrEmpty(href) && href.StartsWith(url))
foreach (var link in links)
html = await httpClient.GetStringAsync(link);
htmlDocument.LoadHtml(html);
var sentences = htmlDocument.DocumentNode.Descendants("p")
.Select(p => p.InnerHtml)
.Where(text => text.Contains("нет"))
foreach (var sentence in sentences)
Console.WriteLine(sentence);