Given two strings a and b, return the length of the longest uncommon subsequence between a and b. If the longest uncommon subsequence does not exist, return -1. An uncommon subsequence between two strings is a string that is a subsequence of one but not the other. A subsequence of a string s is a string that can be obtained after deleting any number of characters from s. For example, "abc " is a subsequence of "aebdc " because you can delete the underlined characters in "aebdc " to get "abc ". Other subsequences of "aebdc " include "aebdc ", "aeb ", and " " (empty string).