Nonlinear elements in circuit

This is orginal picture what I get. The command says "Please find the value of the In current flowing through the nonlinear resistor in the circuit below."


Hi, I'm new in electronic and I have problem with that circuit. I did what I can with that circuit and I'm stuck here. I have to calculate In but I don't know how. Could someone help me ?

What is U1 and Un?
U1 appears to be a current, or is it the voltage across the 200 Ohm resistor?
I assume Un is the resistance of the non linear resistor given as a function of In.