Determine input resistance/impedance
To follow up a little Edgar Brown's statement:

YES. Your notation choose of the output impedance \$R_{in}\$ is infinite. 

Assuming that you ignore \$\lambda\$ means that \$\lambda=0\$. 

I told you about the small signal output impedance \$r_o,\$ which has an equation of its own: 

\$\displaystyle r_o = \frac{1}{\lambda I_D}\$

Since \$\lambda=0,\$ that means 

\$\displaystyle r_o=\frac{1}{0I_D}=\frac{1}{0}=\infty\$

Hence, your output impedance \$R_{in}\$ is \$\infty\$. There is no load resistance nor is there a drain resistance. Your work is finished.