Rain is falling vertically with a speed of \(2\,\text{ms}^{-1}\). A boy from rest starts moving with a constant acceleration of \(2\,\text{ms}^{-2}\) along a straight road holding an umbrella. For the rain to be always parallel to the axis of umbrella, the rate at which the angle of axis of umbrella with the vertical should be changing at time \(t\) is