$f(0)=f(f(5))$
$f(5)=f(f(10))$
$f(10)=f(f(15))$
$f(15)=f(f(20))$
so on…
$f(995)=f(f(1000))$
$f(1000)=f(f(1005))$
now x=1005 >1000 so we return 1001.
so ,
$f(1005)=1001$
$f(1001)=997$
now ,
$f(1000)=f(f(1005))=f(1001)=997$
$f(995)=f(f(1000))=f(997)=f(f(1002))=f(998)=f(f(1003))=f(999)=f(f(1004))=f(1000)=997$
so we can observe that ,$f(995)=f(997)=f(998)=f(999)=f(1000)=997$
say ,we left f(996) behind check for that as well ,
$f(996)=f(f(1001))=f(997)=997$
and so on we can check for all the number till 0 as f(i) where 0<=i <=1001 the value of f(i)=997.
so till x=1001 the value will be 997 . after that the value will be x-4.
