#include <set>
#include <cstdlib>
#include <cstring>
#include <cstdio>

using namespace std;

int best(int from, int to);

set<int>* spreads;
int n;
int* bestchc;
int* ar;

int main()
{
    spreads = (set<int>*)malloc(10000 * sizeof(set<int>));
    while (scanf("%d", &n) != EOF)
    {
	for (int i = 0; i < 10000; i++)
	{
	    spreads[i] = set<int>();
	}
	bestchc = (int*) malloc(n * n * sizeof(int));
	memset(bestchc, 0, n * n * sizeof(int));
	ar = (int*)malloc(n * sizeof(int));
	for (int i = 0; i < n; i++)
	{
	    int spec;
	    scanf("%d", &spec);
	    spec--;
	    ar[i] = spec;
	    spreads[spec].insert(i);
	}
	printf("%d\n", best(0, n - 1));
	free(bestchc);
	free(ar);
    }
    return 0;
}

int best(int from, int to)
{
    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) + 1;
	if (curbest > max)
	    max = curbest;
    }
    bestchc[from * n + to] = max + 1;
    return max;
}