at selection-screen

I have material number and business area in my selection screen. If none of the materials entered are from the business area/areas entered in the selection screen then i want to display the message and exit from there. How do i do that?

AT SELECTION-SCREEN .

IF NOT S_GSBER[] IS INITIAL.

SELECT MATNR GSBER FROM MSEG INTO TABLE IT_MATNR WHERE GSBER IN

S_GSBER AND MATNR IN S_MATNR.

IF IT_MATNR[] IS INITIAL.

WRITE :/ ' Materials entered not in the business area'.

EXIT.

ENDIF.

ENDIF.