There is a superstition in a telephone company: it is believed that a phone number brings good luck if the sum of all its digits is a multiple of 7. You need to write a program that helps clients find the sum of the digits of their phone number.
Write a recursive function sum_of_digits(n) that takes a positive integer and returns the sum of all its digits.
Input format
one is a positive number-the phone number without separators
Output format
one number-the sum of all numbers of the entered number
Hint
There will be no clue here, decide for yourself!