You are given two s: N and K. Lun the dog is interested in strings that satisfy the following conditions:
- The string has exactly N characters, each of which is either 'A' or 'B'.
- The string s has exactly K pairs (i, j) (0 <= i < j <= N-1) such that s[i] = 'A' and s[j] = 'B'.
If there exists a string that satisfies the conditions, find and return any such string. Otherwise, return an empty string.