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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Console;
using static System.Math;

namespace CP
{
    class Atria
    {
        static void Main(string[] args)
        {
            int n = int.Parse(ReadLine());
            string s = ReadLine();
            int ans = 0;
            for (int i = 0; i < n; i++)
            {
                int tmp = 0;
                string X = s.Substring(0, i);
                string Y = s.Substring(i, n - i);
                var xh = new HashSet<char>();
                for (int j = 0; j < X.Length; j++)
                {
                    xh.Add(X[j]);
                }
                var yh = new HashSet<char>();
                for (int j = 0; j < Y.Length; j++)
                {
                    yh.Add(Y[j]);
                }
                foreach (var item in yh)
                {
                    if (xh.Contains(item))
                    {
                        tmp++;
                    }
                }
                ans = Max(ans, tmp);
            }
            WriteLine(ans);
        }

    }

}