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: Time Limit Exceeded)

using System;

namespace atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int a = int.Parse(Console.ReadLine());
            string b = Console.ReadLine();
            int[] list = new int[a];
            int[] list2 = new int[a];
            for (int i = 0; i < a;)
            {
                list[i] = i;
            }
            foreach(int val in list)
            {
                string c = b.Substring(0, val);
                string d = b.Substring(val + 1, a);
                int h = CountOf(d, c);
                list2[val] = h;
            }

            var j = Max(list2);
            Console.WriteLine(j);

        }
        public static int CountOf(string target, params string[] strArray)
        {
            int count = 0;

            foreach (string str in strArray)
            {
                int index = target.IndexOf(str, 0);
                while (index != -1)
                {
                    count++;
                    index = target.IndexOf(str, index + str.Length);
                }
            }

            return count;
        }
        public static T Max<T>(params T[] nums) where T : IComparable
        {
            if (nums.Length == 0) return default(T);

            T max = nums[0];
            for (int i = 1; i < nums.Length; i++)
            {
                max = max.CompareTo(nums[i]) > 0 ? max : nums[i];
                // Minの場合は不等号を逆にすればOK
            }
            return max;
        }
    }
}