Check Box event

Hello Experts,

i have follwoing selection screen.

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-b01.

SELECT-OPTIONS: so_matnr FOR mara-matnr.

PARAMETERS: p_mat1 AS CHECKBOX,

p_mat2 AS CHECKBOX.

SELECTION-SCREEN END OF BLOCK b1.

For my program at a time only one variable is applicable. means if i select p_mat1 then same time i want so_matnr and p_mat2 blank.

Now user decide to select p_mat2 same time on the selection screen p_mat1 and so_matnr should be blank.

At selection-screen output.

when i check ok check box control is not coming here.

Please suggest me some solution.

thanks

amit