(if it's frictionless the length doesn't even matter :) ) It would have the same kinetic energy down as the potential energy up. That is, $mgh=\frac{mv^2}{2}$ or $2gh=v^2$ (the mass doesn't even matter). The result is $\sqrt{2gh}$, so only the height matters really. It is almost 9 (it is $\sqrt{80}=4\sqrt{5}$).