#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];
int rm[5555][5555];
int solve(int l, int r) {
	// [l, r)
	if(l == r) {
		return 0;
	}
	int& ans = dp[l][r];
	if(ans == -1) {
		ans = solve(l+1, r);
//		for(int m = l; m < r; m++) {
		int m = l;
		int Rx = rm[m][r];
		if(Rx != -1 && Rx > l) {
			//printf("l=%d, r=%d, Rx=%d\n", l, r,Rx);
			ans = max(ans, 1+solve(l+1, Rx));
		}
//		}
	}
	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; rm[i][j] = -1; }
		for(int l = 0; l < n; l++) {
			int last = -1;
			for(int r = l+1; r <= n; r++) {
				if(r-1 >= 0 && r-1 >= l && D[r-1] == D[l])
					rm[l][r] = last = r-1;
				else
					rm[l][r] = last;
			}
		}
		printf("%d\n", solve(0, n));
	}
	return 0;
}