dropdown list validation using required field validator with out list value

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
bindgrid();
ds = cci.getcities();
ddlcity.DataSource = ds;
ddlcity.DataTextField = "name";
ddlcity.DataValueField = "id";
ddlcity.DataBind();

ddlcity.Items.Insert(0, "-Select-");

}

set the property of intialvalue="-Select-" for particular dropdownlist required field validator