8. A and B play table tennis, the probability of A winning each game is $\frac{2}{3}$, and the match stops when one person is ahead by two games. The total number of games played is $x_{i}\left(i \in \mathbf{N}_{+}\right)$ with probability $p_{i}$, where $x_{i}<x_{i+1}\left(i \in \mathbf{N}_{+}\right)$, then $S=\sum_{i=1}^{+\infty} x_{i} p_{i}$ $\qquad$ -
$\frac{18}{5}$