Opportunity ID field

Hi everyone,

We use CRM 2015 online. We want create a Field with a unique ID for each opportunity with a prefix depending of the year the opporutinity is created.

Ex.: 15-00001 (created in 2015)

Each time a an opportunity is created we want a +1 counter on the field. Ex.: 15-00002.

Is It possible Something possible with a workflow?

Thanks for your help.