401 UNAUTHORIZED sharepoint

using ASP.NET I have created application.when i upload any file in that that should be stored into sharepoint document library.


protected void btnUpload_Click(object sender, EventArgs e)
{
SPSite mySite = SPContext.Current.Site;
SPWeb myWeb = mySite.OpenWeb();
UploadDocument(myWeb);


}
public void UploadDocument(SPWeb site)
{
if (FileUpload1.HasFile)
{
SPFolder folder = site.GetFolder("Bill");

SPFileCollection files = folder.Files;

Stream fStream = FileUpload1.PostedFile.InputStream; //path of the file to upload

byte[] contents = new byte[fStream.Length];

fStream.Position = 0;

fStream.Read(contents, 0, (int)fStream.Length);

fStream.Close();

string Filename = FileUpload1.FileName;

string URL = SPContext.Current.Site.Url + "/Bill/" + Filename;

SPFile currentFile = files.Add(URL, contents);
}
}