About selection-screen's question?

Dear gurus:

I have declare a screen like below:

SELECTION-SCREEN BEGIN OF BLOCK blk_d WITH FRAME TITLE text-004.

PARAMETERS: p_matnr TYPE resb-matnr. "Material

PARAMETERS: p_bdmng LIKE resb-bdmng. "Request Qty

PARAMETERS: p_meins LIKE resb-meins.

SELECTION-SCREEN END OF BLOCK blk_d.

How to achieve such an effect：

after i input a materical code to p_matnr, display the corresponding unit in p_meins.

Actually, when the user input the qty, them want know the unit of currently material. So i want show the unit in parameter(p_meins) after them input the p_matnr,but before them input the qty.

Message was edited by:

Shao Hui Young