There is a test where one of input strings contain some leading zeroes. It's no big deal to make a workaround, but it leads to inconsistency:

1. Description says we can expect a two strings with number. Not everyone will consider string with digits and leading zeroes a valid number. If we not consider this a number, we need to remove that test.
2. If we consider this a number, then answer (sum of two numbers is undoubtedly a number) with leading zeroes should be accepted or necessity of trimming leading zeroes must be written explicitly in description.