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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

main :: IO ()
main = do
    n <- read <$> getLine
    s <- getLine
    putStrLn $ show $ solve n s


solve :: Int -> String -> Int
solve n s = maximum $ map sameCount $ cutStr n s


sameCount :: (String, String) -> Int
-- aとbに共に含まれる文字の数をカウント
-- ("aab", "bca") -> 2 ('a', 'b'が該当)

sameCount (a, b) =
    length [x | x <- ['a'..'z'], x `elem` a && x `elem` b]


cutStr :: Int -> String -> [(String, String)]
-- 切断した文字列のペアを作成
-- "abc" -> [("", "abc"), ("a", "bc"), ("ab", "c"), ("abc", "")]

cutStr 0 s = [splitAt 0 s]
cutStr n s = (splitAt n s) : cutStr (n - 1) s