How to sum up values from two numeric fields?

How to sum up values from two numeric fields?

Hello, 

I do have a formula field in Zoho CRM and I want to sum up two numeric fields there. I created a formula field and created the fomula like this: 
${Invoices.Field Name 1}+${Invoices.Field Name 1}

But that did not work. Or is it better to use a function to sum up to numeric fields? 


Additional question: 
What's the best way to sum up two values from a picklist that are numbers? 

Thanks for help, 
M.