Need to get runtime as log log n

Function shrink(n)
prev ← n
next ← n-1
while(next > 0)
t ← next
next ← next – (prev-next)*2
prev ← t
return prev
Line 5 of the function is,
“next ← next-(prev-next)*2”.
Instead of multiplying (prev-next) by two, what other operation can be done to (prev-next) to get a runtime of loglog(n), where n is the input to shrink?