#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int max (int a, int b) {
    if (a > b) return a;
    else return b;
}

int main(){
    int i, j, * ans, * duck, n;

    while (1) {
        if (scanf("%i", &n) != 1) break;
        duck = malloc(sizeof(int)* n);
        for (i = 0; i < n; i++) scanf("%i", &duck[i]);
        ans = malloc(sizeof(int) * n * n);

        for(i = 0; i < n; i++){
            *(ans + i*n + i) = 0;
        }
        for (i = 0; i < n - 1; ++i){
            if (duck[i] == duck[i + 1]){
                *(ans + i*n + (i + 1)) = 1;
            } else {
                *(ans + i*n + (i + 1)) = 0;
            }
        }

        for (i = 2; i < n; ++i){
            for (j = 0; (j + i) < n; ++j){
                if (duck[j] == duck[j + i]){
                    *(ans + j*n + i+j) = *(ans + (j + 1)*n + i+j - 1) + 1;
                } else
                    *(ans + j*n + i+j) = max(*(ans + (j + 1)*n + i+j), *(ans + j*n + i+j - 1));
            }
        }
        free(duck);
        free(ans);

        printf("%i\n", *(ans + n - 1));

    }

    return 0;
}