Unable to evaluate the mean length from a list of curve length

Hi everyone.
Brand new grasshopper user, I am trying to compute the average length from a list of segments’ length.
Here is where I am now. I don’t understand why the result of the average box is not a number.
Thanks so much!
average length.gh (7.2 KB)

Flatten Contour or Length outputs or Average input - no need for Number param here.

Oh, by the way, you didn’t internalize that surface in your GH model:

Thank you Joseph, I didn’t know the flatten function.
Problem solved!
Have a great day,

Flatten is rarely the best solution. Many components like ‘Avr’ operate on lists. There can be many lists (branch paths) in a data tree and usually you want to keep it that way. Flatten combines all branches into a single list.