Create AdditivePersistence

Question: 
Have the function AdditivePersistence(num) take the num parameter being passed which will always be a positive integer and return its additive persistence which is the number of times you must add the digits in num until you reach a single digit. For example: if num is 2718 then your program should return 2 because 2 + 7 + 1 + 8 = 18 and 1 + 8 = 9 and you stop at 9.