INITIALIZATION STATEMENT POSITION

Hi here is my code i made:

data l_addition type i.

SELECTION-SCREEN BEGIN OF LINE.
SELECTION-SCREEN COMMENT 8(10) LB1.
PARAMETERS l_num1 TYPE I.
SELECTION-SCREEN END OF LINE.

SELECTION-SCREEN BEGIN OF LINE.
SELECTION-SCREEN COMMENT 8(10) LB2.
PARAMETERS l_num2 TYPE I.
SELECTION-SCREEN END OF LINE.

INITIALIZATION.
LB1 = 'Number 1'.
LB2 = 'Number 2'.

l_addition = l_num1 + l_num2.

write l_addition.

The program when it is excuted it doesn't go futher than the selection-screen

If i put my addition and write statement before initialization it work...

Can you explain me why ?

How INITIALIZATION work with START-OF-SELECTION. ?

Thanks