How does the bounty system work?
I have set a bounty for the question and I have find the answer by myself, so how can I get my reputation back
You don't get it back.
It is gone as soon as you post a bounty.
You are giving up reputation in order for the question to be seen in the "Featured" section.
The bounty you offered essentially is your "payment" for getting your question onto the "featured" tab. While it is there it receives more attention and has a better chance of getting an answer.
If for some reason you don't get any answers, your bounty simply expires and you don't get it back. Similarly, if you find the answer yourself, you don't get the reputation back because you have already reaped the benefits of having your question featured.
So the answer is simply - you can't. Once "paid" for, bounties are non refundable.
You only offered a 50 reputation bounty, that's not too much to get back from upvotes alone! Taking a look at your reputation graph, you even exceed the rep cap yesterday and got over 200! Well done!
I'm sure you'll be able to get those 50 points back really soon!