Ambiguous description.

"Watch out for cases when the number cannot be converted. For example: "1A" cannot be converted with radix 9. For these cases your function should return -1."

This description indicates that the only case when a function returns -1 is when a letter cannot be converted to a number. Description should also mention that there a cases when an input string contains only digits and returns -1. (for example numberRadix("909", 10) == -1).