$\begin{array}{l}
 t = 2\sqrt {{r^2} - {H^2}}  \\ 
  \\ 
 r = \frac{O}{{2\pi }} \\ 
  \\ 
 t = \frac{{\sqrt {{O^2} - 4{H^2}{\pi ^2}} }}{{{\pi ^2}}} \\ 
 \end{array}$