#include <algorithm>
#include <utility>
#include <stdio.h>

using namespace std;

int memo[5001][5001];
int d[5555];

int dp(int a, int b){
	if(a==b) return 0;
	if(a+1==b) return (d[a] == d[b] ? 1 : 0);
	if(memo[a][b] != -1)
		return memo[a][b];
	else{
		memo[a][b] = max(max(dp(a+1,b), dp(a,b-1)), dp(a+1,b-1)+(d[a]==d[b] ? 1:0));
		//printf("%d %d: %d\n", a, b, memo[a][b]);
		return memo[a][b];
	}
}
		


int main(void){
	int n;
	while(scanf("%d", &n) == 1){
		for(int i = 0; i < n; i++)
			scanf("%d", &d[i]);
		for(int i = 0; i < n; i++)
			for(int j = 0; j < n; j++)
				memo[i][j] = -1;
		printf("%d\n", dp(0,n-1));
		//printf("konec\n");
	}
}