Modify Selection screen

HI,

On my selection screen.

I have two parameters

Param1 , Obligatory

Param2

and then two radio buttons

rad1

rad2

My requirement is if i select one radio button param 1 is mandatory

and if i select radiobutton 2 then param1 is not required.it should be grayed out.

When i tried to modify the screen in at selection-screen output

when ever i am selecting second radio button on the selection sreen i am getting a message asking for input for param 1..

How to avoid automatic checks on selection screen.

Thanks

Kiran