Custom Column: Concatenate Text with multiple IF statements

Hey everyone

I am at work and I have to put together some custom columns from my data. I only have basic understanding of the PowerBI languages, any help would be appreciated. 

Here is my starting point. I have two columns with three states. The third column VALUE_MODEL is the custom column, the desired results are given in the fields. The "-" acts as a delimiter between the values from the two columns.

I would compile like this:

VALUE_MODEL = if [TOPICS] <> "" and [LOCAL_BUTTONS] = "" then [TOPICS], else if [TOPICS] <> "" and [LOCAL_BUTTONS] <> "" then [TOPICS]"-"[LOCAL_BUTTONS], else if [TOPICS] = "" and [LOCAL_BUTTONS] = "" then "", else null

I am running into several errors. The first one is a Token Else expected:

Obviously I haven't fully understood the correct syntax here. Any help for a beginner would be highly appreciated. Thank you very much in advance!