How to get VBELN in run time

I have developped a custom driver program and a smartfor layout for sales order confirmation. And configured the program and layout against a output type.

I go to VA02 >Type an order number >

Choose 'Issue output to' from Sales document menu. Then I will select the output type and press print preview button.

In this case my custom driver program is called. My question is during this run time how do I get the VBELN for the current document that I have entered? Based on this VBELN, I am going to fetch the required data.

Regds

Rajesh