Problem 1164. Sum the Digits of a Number

Given an integer, sum the digits repeatedly until you end up with a single value less than 10.

For example, if you add the digits of the number 123456789, you get 1+2+3+4+5+6+7+8+9 = 45, and then 4+5 = 9

The input will be a string of digits. The output should be the scalar value of the repeated sum of digits.