Sum of digits Function

Write a function sum_of_digits that takes an integer value as an argument and returns the sum of digits of a given number. 

Examples

sum_of_digits(123) ➞ 6
sum_of_digits(45) ➞ 9
sum_of_digits(555) ➞ 15

Note: 

• Writing a function is compulsory for this problem. Use "sum_of_digits" as a function name.
  
• Don't forget to return the result.