#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for (int i = (a); i <= (b); i++)
#define FORD(i,a,b) for (int i = (a); i >= (b); i--)
#define REP(i,b) for (int i = 0; i < (b); i++)

const int N = 5000;
int d[N];
int dp[N+1][N+1];

int main() {
    int n;
    while (scanf("%d", &n) == 1) {
        REP(i, n) scanf("%d", &d[i]);
        FOR(len, 2, n) {
            for (int beg = 0; beg + len - 1 < n; beg++) {
                dp[len][beg] = max(dp[len-1][beg], dp[len-1][beg+1]);
                if (len > 2 && d[beg] == d[beg+len-1]) {
                    dp[len][beg] = max(dp[len][beg], dp[len-2][beg+1] + 1);
                }
            }
        }
        printf("%d\n", dp[n][0]);
    }
    return 0;
}