Redirect to another page after Clicking on Ok in Alert Box in ASP.Net

hi all

I have this code in c#

When (ddcity) is empty it goes directly to the previous page

I want to go to the last page after clicking (OK) in the alert

if (string.IsNullOrEmpty(ddcity.Text))
{
    Response.Write("<script>alert('review the Adminstrator');</script>");
    Response.Redirect("main.aspx");
}