Decode a cypher obtained by a digits sum algorithm

I have an algorithm that encodes a number with following rules: -while number has more than one digit, algorithm finds the sum of its digits (e.g. 38 -> 3 + 8 = 11 -> 1 + 1 = 2) -then algorithm returns the sum of all results (from the example above: 38 + 11 + 2 = 51)

So, given a range of numbers, how can I decode it, if it's possible, or just say, that it isn't obtainable by the encoding algorithm (brute force is not an option, since the range could be very wide)