How to parent page name

I have a problem on getting the name of the parent page. here’s my page structure

|_ main
  |_ sub-main1 <-- I'm here!, how do i get the above page name? the (main)
  |_ sub-main2

here’s how I check to see the page name.

if( is_page('sub-main1') OR is_page('main') ){ ... }