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: Runtime Error)

let () = Scanf.scanf "%d" @@ fun n ->
  let s = read_line () in
  Array.init (n + 1) (fun i ->
    Array.init 26 (fun c -> Char.chr @@ Char.code 'a' + c)
    |> Array.to_list
    |> List.filter (fun c ->
        List.exists (fun i -> s.[i] = c) (Array.to_list @@ Array.init i @@ fun j -> j) && List.exists (fun i -> s.[i] = c) (Array.to_list @@ Array.init (n - i) (( + ) i)))
    |> List.length)
  |> Array.fold_left max min_int
  |> Printf.printf "%d\n"