Need help with this its on my homework

Write a program that adds all the digits in an integer. If the resulting sum is more than one digit, keep repeating until the sum is one digit. For example, 123456 has the sum 1+2+3+4+5+6 = 21 which has the sum of 2+1 = 3, your final answer. use the input() function to prompt the user to enter a positive integer. Use while loop