$$$\begin{array}{llll}{E}_{\pi }[R_{t+1}+\gamma G_{t+1}|S_t=s]& =\sum _a\pi (a|s)E_{\pi }[R_{t+1}+\gamma G_{t+1}|S_t=s,A_t=a]& =\sum _a\pi (a|s)\sum _{s^{\prime }}\sum _{r}p(s^{\prime },r|s,a)E_{\pi }[R_{t+1}+\gamma G_{t+1}|S_t=s,A_t=a,S_{t+1}=s^{\prime },R_{t+1}=r]& =\sum _a\pi (a|s)\sum _{s^{\prime }}\sum _{r}p(s^{\prime },r|s,a)(r+\gamma E_{\pi }[G_{t+1}|S_{t+1}=s^{\prime }])\end{array}$$$