Input impedance circuit
Consider definition of input impedance of a circuit:


  The input impedance of an electrical network is the impedance from the source into the network being connected. In other words, the input impedance is the impedance, if placed across the input terminals, that would produce the same voltage across and current through the input terminals as the electrical network being connected. 


In simple words, If you place a voltage source of Vin volts across the input terminals, and the current that flows through it is Iin, then 



Similarly, here Vin=V1



Thus, 

or 

Pole of Zin= -1/2