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 base = int_of_char 'a'
let () =
  Scanf.scanf "%d " @@ fun n  -> 
  let s =  Array.init n (fun _ ->
               Scanf.scanf "%c" (fun x -> x)) in
  let result = ref 0 in
  for i = 0 to (n-1) do
    let left = Array.sub s 0 i in
    let right = Array.sub s i (n-i)  in
    let left' = Array.to_list left in
    let right' = Array.to_list right in
    let num = ref 0 in
    for i = 0 to 26 do
      if List.mem (char_of_int (i+base)) left' = true
         && List.mem (char_of_int (i+base)) right' = true then
        num := 1 + !num
      else
        ()
    done;
    if !num > !result then result := !num
    else ()
  done;
  Printf.printf "%d\n" (!result)