S7 CPU 319F-3 question

We are using S7 CPU 319F-3. When i tried to open some block it is giving following message:

The block can only be read because the software package 'S7 Distributed safety' required for fail-safe programming was not found on your computer '.

My question is how can i get 'S7 Distributed safety' ? Is it free or i need to buy it.

Thanks

buy it

If i'm not wrong the order number is 6ES7833-1FC02-0YA5 for 5.4 Sp3.