Bob is playing a video game that has 7 levels. He starts at level 1, and has probability $p_{1}$ of reaching level 2 . In general, given that he reaches level $j$, he has probability $p_{j}$ of reaching level $j+1$, for $1 \leq j \leq 6$. Let $X$ be the highest level that he reaches. Find the PMF of $X$ (in terms of $p_{1}, \ldots, p_{6}$ ).