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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.Map;

public class Main{

	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String s = br.readLine();
		String[] fh = s.substring(0,  s.length()/2).split("");
		String[] sh = s.substring(s.length()/2,  s.length()).split("");
		int ans = 0;
		Map<String, Boolean> map = new HashMap<>();

		for(int i = 0; i < fh.length; i++) {
			if(map.get(fh[i]) == null) map.put(fh[i], false);
		}
		for(int j = 0; j < sh.length; j++) {
			if(map.get(sh[j]) == null) {
				map.put(sh[j], false);
			} else if(map.get(sh[j]) == false){
				map.put(fh[j], true);
			}
		}

		for(Boolean value: map.values()) {
			if(value == true) ans++;
		}
		System.out.println(ans);
	}

}