#include <iostream>
#include <vector>
#define LL long long
#define MAX 6000

using namespace std;

vector<vector<LL>> dyn(MAX, vector<LL>(MAX, 0));
vector<LL> vstup(MAX);

int main() {
	std::iostream::sync_with_stdio(false);
	LL N;
	while (cin >> N) {
		for (int i = 0; i < N; ++i) {
			cin >> vstup[i];		
		}

		for (int k = 0; k < N; ++k) {
			for (int l = 0; l < N - k; ++l) {
			    LL i = l;
			    LL j = l + k;
			    
				if (k == 0) {
					dyn[i][j] = 0;
					continue;			
				} else {
				    if (vstup[i] == vstup[j]) {
				        dyn[i][j] = 1 + dyn[i+1][j-1];
				    } else {
				        dyn[i][j] = max(dyn[i+1][j], dyn[i][j-1]) ;
				    }
				}
			}		
		}
		
		cout << dyn[0][N-1] << endl;
	}

	return 0;
}