Average record values of sub-form

Average record values of sub-form

Hello I have a Form called Opportunity with a sub-form called Voting, containing fields called Number.
I would like to have a field showing the average of the voting numbers for that particular opportunity.
So I created a formula filed on the Opportunity with the following:
  1. input.Opportunity.Voting[ID != 0].avg(Number);
I get a syntax error.
Could you please help.

Thank you