$S_{1}, S_{2}, ..., S_{16}$ play in a tournament. They are divided into **eight pairs** at random. From each pair a winner is decided on the basis of a game played between the two players of the pair. Assume that all the players are of equal strength.

Find the probability that exactly one of the two players $S_{1}$ and $S_{2}$ is **among the eight winners**.

The probability is in the form $\frac{a}{b}$. Find

a + b.

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