一个 「开心字符串」定义为：

A happy string is a string that:

对任一由 n 个小写英文字母组成的字符串 word ，我们可以定义一个 n x n 的矩阵，并满足：

We define the lcp matrix of any 0-indexed string word of n lowercase English letters as an n x n grid such that:

给你两个字符串 s1 和 s2 ，两个字符串的长度都为 4 ，且只包含 小写 英文字母。 你可以对两个字符串中的 任意一个 执行以下操作 任意 次：

You are given two strings s1 and s2, both of length 4, consisting of lowercase English letters. You can apply the following operation on any of the two strings any number of times:

给你两个字符串 s1 和 s2 ，两个字符串长度都为 n ，且只包含 小写 英文字母。 你可以对两个字符串中的 任意一个 执行以下操作 任意 次：

You are given two strings s1 and s2, both of length n, consisting of lowercase English letters. You can apply the following operation on any of the two strings any number of times:

给你两个字符串，str1 和 str2，其长度分别为 n 和 m 。

You are given two strings, str1 and str2, of lengths n and m, respectively. A string word of length n + m - 1 is defined to be generated by str1 and str2 if it satisfies the following conditions for each index 0 <= i <= n - 1: