Text #2 for internship at Evo

This is my execution of the test task #2

I have made a couple of assumptions prior to planning the project:

1. The input string is valid and no spaces between the digit symbols, i.e. "37" but no "3 7"

2. I have made the limit for the length of the digit - 12 symbols; It is not hard to expand the vocabulary for trillion and sextillion etc., but ordinary calculators have same digit length;

3. I skipped for now more complex math signs like square root, logarithm or power function;