//include <iostream>
#include<unordered_map>
#include <iomanip>
#include <cstdio>
#include<vector>
#include <cmath>
#define ll long long
using namespace std;

int n,m, ans;
char c;

vector<vector<int>> sous;
vector<vector<int>> tab;
vector<int> in;
//vector<int> last;

int main()
{
    //ios::sync_with_stdio(false);
    //cout << setprecision(10);
   while(scanf("%d",&n) != -1)
   {
        //init
        sous = vector<vector<int>>(10001, vector<int>());
        tab = vector<vector<int>>(n+2, vector<int>(n+2));
        in = vector<int>(n);
        //last = vector<int>(n,-1);
        //scanf("%d", &m);

        for(int i = 0; i < n; i++) {
            int d;
            scanf("%d", &d);
            in[i] = d;
            sous[d].push_back(i);
        }

        //alg
        for(int i = 0;i <= n; i++) {
            tab[0][i] = 0;
        }

        for(int i = 1;i <= n; i++) {
            tab[i][n] = 0;
            for(int j = n-1; j >= i; j--) {
                int m = max(tab[i][j+1], tab[i-1][j]);
                if (in[i-1] == in[j])
                    m = max(m, tab[i-1][j+1]+1);
                tab[i][j] = m;
                //printf("%d\n", m);
            }
        }
        int best = 0;
        for(int i = 0;i <= n; i++) {
            best = max(best, tab[i][i]);
        }
        printf("%d\n", best);
   }
    return 0;
}