How to input the variables for F(x,y)??

Dear All Grasshopper experts

I am new in this and this might sound a very stupid question to you but I have been struggleing all day in trying to input the following variables into the component F(x,y).

I double clicked the icon and the Expression Designer poped out pompting me to input the variables.

I need to input x1=cos(x) cos(y)

but it would only allow me to input either one but not both. If I try to put an "&" or "+" in between the 2 variables, it accptes it but the resulting 3D surface becomes very strange without much evidence of logic.

I am actually going through the Chapter 3 of Parametric Architecture book on page 58.

can any one be so kind to help me in this issue? what am I doing wrong?

sorry for the stupid question and thanks in advance!!

yutaka