While reading about reinforcement learning, I have come across the following expression for expected rewards in terms of a summation, the denominator of which I am not able to account for.

The formula given is:

[![enter image description here][1]][1]

According to what I understand, the formula should have been correct without the denominator (that I have highlighted). How is this formula correct?

Source of image: Andrew G and Sutton's book on RL.



  [1]: https://i.sstatic.net/Mc7Te.png