import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Mugs {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        int chars = Integer.parseInt(br.readLine());

        //Na pozici 20 length
        List<int[]> charPoints = new ArrayList<>();

        int bestLenght = 1;

        for (int i = 0; i < chars; i++){
            char c = (char)br.read();

            for (int[] mp:charPoints) {
                mp[c - 'a']++;
                mp[20]++;
                if(checkValid(mp)){
                    if(mp[20] > bestLenght){
                        bestLenght = mp[20];
                    }
                }
            }
            
            int[] temp = new int[21];
            temp[c - 'a']++;
            temp[20]++;
            charPoints.add(temp);
        }


        System.out.println(bestLenght);

    }

    public static boolean checkValid(int[] charOccur){
        int odd = 0;

        for (int i = 0; i < 20; i++) {
            odd += charOccur[i] % 2 == 1 ? 1 : 0;
        }

        return odd <= 1;
    }
}