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.chomp.to_i
s = gets.chomp
ans = 0
0.upto(n) do |i|
  count = 0
  as = []
  0.upto(i) do |j|
    if !as.include?(s.split("").at(j)) then
      as.push(s.split("").at(j))
    end
  end
  bs = []
  i.upto(n) do |j|
    if !bs.include?(s.split("").at(j)) then
      bs.push(s.split("").at(j))
    end
  end
  as.each do |char|
    if bs.include?(char) then
      count += 1
    end
  end
  if ans < count then
    ans = count
  end
end
puts ans