#include <iostream>
#include <set>
#include <cstdlib>
#include <cstring>

using namespace std;

int best(int from, int to, int* bestchc, set<int>* spreads, int n, int* ar);

int main()
{
    ios::sync_with_stdio(false);
    int n;
    while (cin >> n)
    {
	set<int>* spreads = (set<int>*)malloc(10000 * sizeof(set<int>));
	for (int i = 0; i < 10000; i++)
	{
	    spreads[i] = set<int>();
	}
	int* bestchc = (int*) malloc(n * n * sizeof(int));
	memset(bestchc, 0, n * n * sizeof(int));
	int* ar = (int*)malloc(n * sizeof(int));
	for (int i = 0; i < n; i++)
	{
	    int spec;
	    cin >> spec;
	    spec--;
	    ar[i] = spec;
	    spreads[spec].insert(i);
	}
	cout << best(0, n - 1, bestchc, spreads, n, ar) << endl;
    }
    return 0;
}

int best(int from, int to, int* bestchc, set<int>* spreads, int n, int* ar)
{
    //cout << "best" << from << " " << to << endl;
    if (bestchc[from * n + to] != 0)
    {
    	return bestchc[from * n + to] - 1;
    }
    if (to - from < 1)
    {
        return 0;
    }
    int max = 0;
    for (int i = from; i < to + 1; i++)
    {
        int spec = ar[i];
	int first = *(spreads[spec].lower_bound(from));
	set<int>::iterator lastit = spreads[spec].upper_bound(to);
	lastit--;
	int last = *lastit;
	if (first == last)
	{
	    continue;
	}
	int curbest = best(first + 1, last - 1, bestchc, spreads, n, ar) + 1;
	if (curbest > max)
	    max = curbest;
    }
    bestchc[from * n + to] = max + 1;
    return max;
}