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 Ruby (2.3.3) (judge verdict: Accepted)

$N = gets.to_i
$S = gets.chomp
$ans = 0
def solve()
    l = $S.size
    (l-1).times{|i|
        tmp = 0
        a = $S[0..i]
        b = $S[(i+1)..l]
        table = {}
        a.each_char{|s|
            table[s] = 1 if table[s] == nil
        }
        b.each_char{|s|
            tmp = tmp + 1 if table[s] == 1
            table[s] = 0
        }
        $ans = [$ans,tmp].max
    }
end

solve
puts $ans