Disableing the Field ?

Hello ,

I am having 2 input fields in which one is dropdown having YES and NO.

And the second field should get automatically disable if i am choosing NO in my first field.

For this requirement how i have to code.

Till now my idea is in PAI...

My code

ZORMA-CISWK = 'NO'. "first field

IF SY-SUBRC = 0.

LOOP AT SCREEN.

IF SCREEN-NAME = 'ZORMA-YISWK'. "field which has to be disabled

SCREEN-ACTIVE = 1.

SCREEN-INPUT = 0.

SCREEN-OUTPUT = 1.

MODIFY SCREEN.

ENDIF.

ENDLOOP.

ENDIF.

But this is not working. Help me to achieve this...

Thks