The inital water level in a tank is $4\:m$. When the value located at the bottom is opened, the rate of change of water level ($L$) with respect to time ($t$) is, $\frac{dL}{dt}=-k\sqrt{t}$, where  $k=0.6\:m\:s^{-3/2}$. The level of water (in $m$) in the tank at time $0.5\:s$ after opening the valve is _______________ (rounded off to second decimal place).