How many choices for this combination?

Question

Suppose that we a string contains n multiplicates of '-1' and '+1', we denote it $A = {\{a_1,a_2,...,a_{2n}\}}$.

Define $S_k = \sum_{i=1}^{k}a_i$. If, $\forall k\leq 2n, S_k \geq 0$, then how many choices for the string?