Hi Anna! What if you change the "f" to an "x"? In the newer versions of Grasshopper "x" is the default letter to assign a value. Alternatively, you could use the "Expression" component see image Let me know if this works.

It is an error here. "f*" is not recognizable by program

Hi Mohamed, your image didn't get posted, unfortunately. Could you repost it, so I can have a look? thanks! Arie