Given two strings, str1 and str2, find the index of the extra character that is present in only one of the strings.

Note: If multiple instances of the extra character exist, return the index of the first occurrence of the character in the longer string.

Constraints:

• $0 \leq$ str1.length, str2.length $\leq 1000$
• Either str2.length $=$ str1.length + 1, or, str1.length $=$ str2.length + 1
• The strings consist of lowercase English letters.