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.
Nice Problem.
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