Gambler's Ruin Problem

A gambler starts with an initial fortune of $i$ dollars. On each successive game, the gambler wins $1 with probability $p$ ($0 < p < 1$), or loses $1 with probability $q = 1 - p$. He will stop if he either accumulates $N$ dollars or loses all his money. What is the probability that he will end up with $N$ dollars?