istudiez pro - do i really have to purchase again?

hey all,

i have the istudiez pro app on my iPad - which i have paid for. now, i want to get in on my mac too, but when i enter the mac app store, i'm requested to purchase it (again?). can i get the app on my mac for free, or is there no relation between the too, in which case if i want it i need to buy it again?