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: Wrong Answer)

n = gets.to_i
s = gets.chomp

result = 0

max =
  if n > 50
    25
  else
    n/2
  end

(1..(n-1)).each do |z|
  x = s.slice(0, z)
  y = s.slice(z, n)

  if n-z < result
    break
  end

  x_h = {}
  x.each_char do |c|
    x_h[c] = true
  end

  y_h = {}
  y.each_char do |c|
    y_h[c] = true
  end

  keys = x_h.keys & y_h.keys
  cnt = keys.length
  if cnt > result
    result = cnt
  end

  if cnt >= max
    break
  end
end

puts result