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)

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

t={}
for i=1,n do
    alphabet=s:sub(i,i)
    if not t[alphabet] then
        t[alphabet]={}
    end
    table.insert(t[alphabet],i)
end

max=0
for i=1,n do
    counter=0
    for alphabet,_ in pairs(t) do
        if #t[alphabet]>1 then
            if t[alphabet][1]<=i and i<=t[alphabet][#t[alphabet]] then
                counter=counter+1
            end
        end
    end
    max=math.max(counter,max)
end
print(max)