Hitting time Distribution of a Gaussian Random Walk

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I am trying to find out the exponential decay rate of the Probability $Pr(T>n)$ where $T$ is the first hitting time of a gaussian random walk with i.i.d random variables i.e. $S_{n}=\sum_{i=1}^{n}X_{i}$ where $X_{i}$'s are iid and which have non-zero means and are distributed as $\mathcal{N}(m,2m/N)$. The hitting time is respect to a threshold $B$. I can find upper bounds and lower bounds of the above mentioned probability. Does there exist an exact distribution for the same?