Counting down from $13$ by $2$ takes you to

$\begin{array}{cccccccccccccc} &&&&&-1&&-1&&&&& \\ & &&&&\curvearrowleft &&\curvearrowleft&& &&&&\\ 9&&10&&\underline{11}&&12&&\underline{13}&&14&&15&& \\ |&&|&&|&&|&&|&&|&&| \\ \hline \end{array}$

So the answer is $11$.