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 OCaml (4.02.3) (judge verdict: Accepted)

let n = read_int ()
let s = read_line ()
let ans = ref 0
let _ = for i = 0 to n - 1 do let c = ref 0 in
  for j = Char.code 'a' to Char.code 'z' do if String.(contains (sub s 0 i) @@ Char.chr j && contains (sub s i @@ n - i) @@ Char.chr j) then incr c done;
  ans := max !ans !c done; Printf.printf "%d\n" !ans