When you purchase OS X, you will be able to download as many times as you want for free. You won't have to pay to reinstall OS X Mountain Lion
but I do not buy os x, I've got since purchased
As you got OS X with your computer, OS X is registered to your Apple ID and it appears as purchased (although you didn't purchase it). That's why you can reinstall OS X through Internet Recovery
on macbook,press comand and R,choose a wifi,download,half loading,there s an error,
Retrieving data ...