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 Lua (5.3.2) (judge verdict: Accepted)

local n = io.read("*n", "*l")
local s = io.read()

local function check(i, str)
  local left = str:sub(1, i)
  local right = str:sub(i + 1, #str)
  local t = {}
  for i = 1, i do
    t[left:sub(i, i)] = true
  end
  local c = 0
  for k, v in pairs(t) do
    if right:find(k) then
      c = c + 1
    end
  end
  return c
end

local cnt = 0
for i = 1, n - 1 do
  cnt = math.max(cnt, check(i, s))
end
print(cnt)