#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

int a[5001], N, dp[5001][5001];
int solve(int i, int j);

int main() {
	while (cin >> N) {
		for (int i = 0; i < N; i++) {
			cin >> a[i];
		}
		memset(dp, -1, sizeof(dp));
		cout << solve(0, N - 1) << endl;
	}
}

int solve(int i, int j) {
	if (dp[i][j] != -1) return dp[i][j];
	if (i >= j) return 0;

	int pom = 0;
	if (a[i] == a[j]) {
		pom = 1 + solve(i + 1, j - 1);
	}

	return dp[i][j] = max(max(solve(i, j - 1), solve(i + 1, j)), pom);
}