public class Rate
        {
            public string no { get; set; }
            public string effectiveDate { get; set; }
            public string mid { get; set; }
        }

        public class NBPapi
        {
            public string table { get; set; }
            public string currency { get; set; }
            public string code { get; set; }
            public List<Rate> rates { get; set; }

        }

        public static float wylicz(string kurs)
            {
                WebRequest request = HttpWebRequest.Create(
                    "http://api.nbp.pl/api/exchangerates/rates/A/" + kurs + "/?format=json&fbclid=IwAR1I1KkXoCFuiJ3HCIOJ4uPqVH_WQLimmHQN0ZOjcHihLLpVm0bhXyoobjA");

                WebResponse response = request.GetResponse();
                StreamReader reader = new StreamReader(response.GetResponseStream());

                string jsonik = reader.ReadToEnd();

                NBPapi nbpapi = Newtonsoft.Json.JsonConvert.DeserializeObject<NBPapi>(jsonik);
                string usdCurr = nbpapi.rates.ElementAt(2).ToString();

                return float.Parse(usdCurr);
            }

    }