#include <bits/stdc++.h>

using namespace std;
using lli = int64_t;

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

	return (dp[i][j] = max ({solve (dp, a, i+1, j), solve (dp, a, i, j-1), solve (dp, a, i+1, j-1) + (a[i] == a[j]?1:0)})); 
}
int main () {
	for (;;) {
		lli n;
		cin >> n;
		if (!cin) break;
		auto a = vector<int> (n);
		for (auto & ai : a)
			cin >> ai;
		auto dp = vector<vector<int>> (n, vector<int> (n, -1));
		cout << solve (dp, a, 0, n-1) << '\n';
	}
	return 0;
}