using System;

using System.Collections.Generic;

using System.Net;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(PhoneRegionExtractor.GetRegion(9173006556));

    }

    public static class PhoneRegionExtractor

    {

        private static Dictionary<long, string> _cache;

        public static string GetRegion(long phone)

        {

            if (!validate(phone))

            {

                throw new ApplicationException("Invalid phone number");

            }

            string result = GetOrAddFromCache(_cache, phone);           

            return result;

        }

        private static bool validate(long phone)

        {

            Regex Rex = new Regex("[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]");

            return Rex.IsMatch(phone.ToString());

        }

        private static string GetFromWebservice(long phone)

        {

            var regex = new Regex("(\"region\".*\"name\":\"(?<region>.*)\",\"okrug\")");

            var client = new WebClient();

            string s = client.DownloadString("https://htmlweb.ru/json/mnp/phone/" + phone);

            var match = regex.Match(s);

            return match.Success ? match.Groups["region"].Value : string.Empty;

        }

        private static string GetOrAddFromCache(Dictionary<long, string> cache, long number)

        {

            if (cache == null)

            {

                cache = new Dictionary<long, string>(); 

            }

            if (cache.ContainsKey(number))

            {

                return cache[number];

            }

            string result = GetFromWebservice(number);

            cache.Add(number, result);

            return result;

        }

    }

}