selection-screen problem

Hi all,

I have two radiobuttons and single parameter on the selection screen.I want the parameter field obligatory when first radiobutton is clicked and not obligatory when second button is clicked.its working fine when i put some values in parameter otherwise its giving me error that enter value in required fiels.

below is the code ,Please suggest me what can i do..

parameters: r1 radiobutton group rad1 default 'X' user-command radio,

r2 radiobutton group rad1.

parameters: p1 type matnr modif id a.

at selection-screen output.

IF R1 = 'X'.

loop at screen.

if screen-group1 = 'A'.

screen-input = 0.

MODIFY SCREEN.

endif.

endloop.

ENDIF.

IF R2 = 'X'.

loop at screen.

if screen-group1 = 'A'.

screen-required = 0.

MODIFY SCREEN.

endif.

endloop.

ENDIF.

Regards

Lalit