#include <bits/stdc++.h>

#define REP(i, n) for(int i = 0 ; i< (int)n; ++i)

using namespace std;



bool testcase(){
	int n;

	if(!(cin >> n))return false;

	vector<int> values(n);
	for(int i = 0; i < n; i++)
		cin >> values[i];

	vector<vector<int>> dp(n+1);
	for(auto& x : dp)
		x.resize(n+1);

	for(int d = 0; d < n; d++)
	{
		for(int i = 0; i < n - d; i++)
		{
			int l = i;
			int r = i + d;
			if(d == 0)
				dp[l][r] = 0;
			else
			{
				if(values[l] == values[r])
					dp[l][r] = dp[l+1][r-1] + 1;
				else
					dp[l][r] = max(dp[l+1][r], dp[l][r-1]);
			}
		}
	}
	/*
	for(int i = 0; i < n; i++)
		{
		for(auto& x : dp[i])
			cerr << x << " ";
		cerr << endl;
		}
	*/
	cout << dp[0][n-1] << endl;

	return true;
}


int main(){
	while(testcase());
	return 0;
}