Easy question i think >>?

hello >>>>

i have easy question i think.<<

i make a dynamic text < it value is 5 for example < and it instance name is “a”
and i have a hits full screen <
i want whene i press on the hits < the number of dynamic text - 1 " in this code :

on (onPress){
a.text = a.text - 1 ;
}

but not here problem >>???
i want whene the dynamic text value is = 0 > stop >
i mean < i dont want any numer under zero >>> when dynamic text value is zero

and i,m veryyyyyyyyyyyy sorry < and i waite any hero help me >

goo9oooooooooo9o9o9o9od byy

samo