Strings s1 and s2 are k-similar (for some non-negative integer k) if we can swap the positions of two letters in s1 exactly k times so that the resulting string equals s2. Given two anagrams s1 and s2, return the smallest k for which s1 and s2 are k-similar. Example 1: Input: s1 = "ab", s2 = "ba" Output: 1 Example 2: Input: s1 = "abc", s2 = "bca" Output: 2 Example 3: Input: s1 = "abac", s2 = "baca" Output: 2 Example 4: Input: s1 = "aabc", s2 = "abca" Output: 2 Constraints: 1 <= s1.length <= 20 s2.length == s1.length s1 and s2 contain only lowercase letters from the set {'a', 'b', 'c', 'd', 'e', 'f'}. s2 is an anagram of s1.