Random Walk on $\mathbb{C}$

Question

Let $\phi_j$ be independent and uniformaly distrubited random variables on $[0, 2\pi)$.
Then we define the random Walk on $\mathbb{C}$ as follows: $$\sum_{j=1}^nae^{i\phi_j}$$ where $a \in \mathbb{R}$. Is possible to calculate the probability that $|\sum_{j=1}^nae^{i\phi_j}| \le c$ accurately?