Problem:
Score : 200 points

Problem Statement

You are given a string S of length N consisting of lowercase English letters.
We will cut this string at one position into two strings X and Y.
Here, we would like to maximize the number of different letters contained in both X and Y.
Find the largest possible number of different letters contained in both X and Y when we cut the string at the optimal position.

Constraints

2 \leq N \leq 100

|S| = N

S consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the largest possible number of different letters contained in both X and Y.

Sample Input 1

6
aabbca

Sample Output 1

2

If we cut the string between the third and fourth letters into X = aab and Y = bca, the letters contained in both X and Y are a and b.
There will never be three or more different letters contained in both X and Y, so the answer is 2.

Sample Input 2

10
aaaaaaaaaa

Sample Output 2

1

However we divide S, only a will be contained in both X and Y.

Sample Input 3

45
tgxgdqkyjzhyputjjtllptdfxocrylqfqjynmfbfucbir

Sample Output 3

9

Solution in Common Lisp (SBCL 1.1.14) (judge verdict: Accepted)

(defun duplicate-char-counter (str separator)
  (labels ((duplicate-char-counter (char-list cnt)
             (if char-list
                 (duplicate-char-counter (cdr char-list)
                                         (if (find (car char-list) str :start separator :test #'char=)
                                             (1+ cnt)
                                             cnt))
                 cnt)))
    (let ((char-list (coerce (remove-duplicates (subseq str 0 separator)) 'list)))
      (duplicate-char-counter char-list 0))))

(princ (loop for separator from 1 upto (1- (read))
             with input-str = (read-line)
             maximize (duplicate-char-counter input-str separator)))