Find the length of the longest subsequence with first K alphabets having same frequency

Given string str with uppercase characters and an integer K, the task is to find the length of the longest subsequence such that the frequency of the first K alphabet is the same.

Examples: 

Input: str = "ACAABCCAB", K=3 
Output: 6 
Explanation: One of the possible subsequences is "ACABCB".

Input: str = "ACAABCCAB", K=4 
Output: 0 
Explanation: Since the string does not contain 'D', no such subsequence can be obtained.  

Approach: 
Traverse the string and find the least frequent of the first K alphabets. Once found, (frequency of that element) * K gives the desired result.
Below is the implementation of the above approach:

6