Problem Statement  
Given a string s, its roughness is calculated as follows: Let c1 be the letter that appears most frequently in s, and let c2 be the letter that appears least frequently (c2 must appear at least once). The roughness of s is the number of occurrences of c1 minus the number of occurrences of c2. You are allowed to modify s by erasing between 0 and n characters, inclusive (see example 1 for clarification). Return the minimum possible roughness that can be achieved by such a modification.  
Definition  
 
Constraints  
  s will contain between 1 and 50 characters, inclusive.  
  s will contain only lowercase letters ('a''z').  
  n will be between 0 and m1, inclusive, where m is the number of characters in s.  
Examples  
0)  
 
1)  
 
2)  
 
3)  
 
4)  
 
5)  
