A single sentence symbol wff has $s=1$ sentence symbol and $c=0$ binary connective symbols. Now, if $\alpha$ and $\beta$ are two wffs each having sentence symbols ($n$ and $m$ , respectively) one more than binary connective symbols, the negation of $\alpha$ does not change the number of either sentence symbols or binary connective symbols, and any wff $(\alpha\star\beta)$ , where $\star$ is one of the binary connective symbols, has $n+m$ sentence symbols and $n-1+1+m-1=n+m-1$ binary connective symbols. Using the induction principle outlined in the textbook, we are done.