Given two string arrays `words1` and `words2`, return the number of strings that appear exactly once in each of the two arrays.

Example 1:

    Input: words1 = ["leetcode","is","amazing","as","is"], words2 = ["amazing","leetcode","is"]
    Output: 2
    Explanation:
    - "leetcode" appears exactly once in each of the two arrays. We count this string.
    - "amazing" appears exactly once in each of the two arrays. We count this string.
    - "is" appears in each of the two arrays, but there are 2 occurrences of it in words1. We do not count this string.
    - "as" appears once in words1, but does not appear in words2. We do not count this string.
    Thus, there are 2 strings that appear exactly once in each of the two arrays.

Example 2:

    Input: words1 = ["b","bb","bbb"], words2 = ["a","aa","aaa"]
    Output: 0
    Explanation: There are no strings that appear in each of the two arrays.

Example 3:

    Input: words1 = ["a","ab"], words2 = ["a","a","a","ab"]
    Output: 1
    Explanation: The only string that appears exactly once in each of the two arrays is "ab".

Constraints:

- `1 <= words1.length, words2.length <= 1000`
- `1 <= words1[i].length, words2[j].length <= 30`
- `words1[i]` and `words2[j]` consists only of lowercase English letters.