Expected stopping time for a biased random walk

Question

Let $p \in [0,1]$ and $\{X_i\}_{i =1}^\infty$ be a sequence of $\{-1, 1\}$-valued i.i.d random variables taking value 1 with probability $p$. We can consider the biased random walk $S_t$ defined at time $t$ by $$ S_t = \sum_{i=1}^t X_t. $$ Let $d > 0$ and define a stopping time $\tau$ by $\tau = \min \{ t \in \mathbb{Z}^+ : S_t \in \{-d, d\} \}$. What is the expectation of $\tau$?