validation on selection screen field

Hello Experts,

I have two radiobuttons on my selection-screen and each having its own parameters (obligatory).

SELECTION-SCREEN BEGIN OF BLOCK rad1 WITH FRAME TITLE title.

PARAMETERS: p_r1 RADIOBUTTON GROUP gr1 USER-COMMAND flag,

            p_file TYPE dxfile-filename MODIF ID b1,

            p_r2 RADIOBUTTON GROUP gr1.

SELECT-OPTIONS: s_lifnr FOR gs_final-lifnr MODIF ID b2,

                s_bukrs FOR gs_final-bukrs MODIF ID b2.

SELECTION-SCREEN END OF BLOCK rad1.

=> I want to make all the paramerts as obligatory. if i execute the program with having blank parameters(obligatory) then program should generate error messge as 'fill required field'.

After performing above check on screen param , i shoulnt skip from the selection screen

=> second

  by default screeen will be having p_1 as checked and p_r2 in disable mode

  suppose p_file is blank and p_r1 checked, if i try to check / enable p_r2 radiobutton where my p_file is blank then p_r2 shouldnt enable in that case and generate error message