Looking for a combinatorial problem definition

We are given "N" pairs of pare开发者_如何学编程ntheses, ie "N" opening parenthesis "(" and "N" closing parenthesis ")". We are asked to find the number of ways to make Sequence of 2N parentheses that are GOOD, i.e. we dont close before opening.

I need to find a definition for GOOD Sequences that i can use for the rest of the problem.

---

Catalan numbers!