A system with the transfer function $$\frac{{Y\left( s \right)}}{{X\left( s \right)}} = \frac{s}{{s + p}}$$   has an output $$y\left( t \right) = \cos \left( {2t - \frac{\pi }{3}} \right)$$    for the input signal $$x\left( t \right) = p\cos \left( {2t - \frac{\pi }{2}} \right).$$     Then, the system parameter 'p' is