#include <bits/stdc++.h>
#define REP(A,B) for(int (A)=0;(A)<(B);(A)++)
#define pb push_back
using namespace std;
int D[5555];
int dp[5555][5555];
set<int> S[5555];
int solve(int l, int r) {
	// [l, r)
	if(l == r) {
		return 0;
	}
	int& ans = dp[l][r];
	if(ans == -1) {
		ans = 0;
		for(int m = l; m < r; m++) {
			auto it = S[D[m]].upper_bound(m);
			if(it != S[D[m]].end() && *it < r) {
				//printf("beru kachnu na m=%d, rightmost je %d [r=%d]\n", m, *it, r);
				int rightmost = *it;
				ans = max(ans, 1+solve(m+1, rightmost));	
			}
		}
	}
	return ans;
}
int main()
{
	int n,k;
	while(scanf("%d", &n) == 1) {
		REP(i, n) scanf("%d", D+i);
		REP(i, n+1) REP(j, n+1) dp[i][j] = -1;
		REP(i, n+1) S[i].clear();
		REP(i, n) {
			S[D[i]].insert(i);
		}
		printf("%d\n", solve(0, n));
	}
	return 0;
}