Could you please answer my question, thank you very much!

here are some laws about legal bracket strings:

• an empty string is a legal bracket string
• if A is a legal bracket string, so (A) is a legal bracket string
• if A is a legal bracket string and B is a legal bracket string, so AB is a legal bracket string

And I want to know that what is the minimum edits that letting a bracket string to be legal?

Thank you so much!