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)

require 'pp'

# 入力値を文字の配列で返却する
def gets_nsp_list()
  gets.chomp.chars
end

# 入力値を数値で返却する
def gets_i()
  gets.chomp.to_i
end

N = gets_i
S = gets_nsp_list

max = 0
(1...N).each do |n|
  left  = S[0,n]
  right = S[n,N]
  max = [max, (left.uniq & right.uniq).size].max
end
puts max