#include <bits/stdc++.h>

typedef long long ll;
typedef long double ld;

using namespace std;

#define rep(i, a, n) for (int i = (a); i < (n); i++)
#define per(i, a, n) for (int i = (n) - 1; i >= (a); i--)
const int N = 5010;

int dp[N][N];

int main(void) {
	ios::sync_with_stdio(false);
	int n;
	while(cin>>n) {
        memset(dp, 0, sizeof(dp));
        vector<int> a(n);
        rep(i,0,n) cin >> a[i];

        rep(to,0,n) {
            per(from,0,to) {
                dp[from][to] = max(dp[from][to], dp[from][to-1]);
                dp[from][to] = max(dp[from][to], dp[from+1][to]);
                if (a[to] == a[from]) {
                    dp[from][to] = max(dp[from][to], dp[from+1][to-1] + 1);
                }
            }
        }

        cout << dp[0][n-1] << endl;
    }

	return 0;
}