import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.InputStreamReader;

public class Gallery {

    static int in[];


    static int getScore(int s, int e, int sc) {
        int max = sc;

        int i;
        for (i = s; i < e-1; i++) {
            for (int j = e; j > i; j--) {
                if (in[i] == in[j]) {

                    int score = getScore(i+1,j-1, sc+1);
                    if (score > max) {
                        max = score;
                    }
                    if (max - sc >= (j-i)/2 ) {
                        return max;
                    }
                    break;
                }
            }
        }
        return max;
    }


    public static void main(String... args) throws Exception {
        // try (BufferedReader reader = new BufferedReader(new FileReader(new File("gallery.in")))) {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {

            String line;
            while ((line = reader.readLine()) != null) {
                int len = Integer.parseInt(line);
                String input[] = reader.readLine().split(" ");

                in = new int[len];

                for (int i = 0; i < len; i++) {
                    in[i] = Integer.parseInt(input[i]);
                }

                System.out.println(getScore(0, len-1, 0));


            }
        }
    }
}