Get only custom lists from sharepoint lists

[sharmileekumari]

Can anyone suggest me how can I retrieve only custom lists from sharepoint site?

I'm using this code to get sharepoint lists but it gives me all lists (custom lists+built in lists):

SPWeb objWeb = SPContext.Current.Web;
  SPListCollection lists = objWeb.Lists;
  ComboBox1.Items.Clear();

  foreach (SPList list in lists)
    {
      ComboBox1.Items.Add(list.Title);                
    }

How can I get only custom lists?

Thank You,