#include <bits/stdc++.h>
using namespace std;

#define PB push_back
#define CL(A, I) (memset(A, I, sizeof(A)))
#define D(X) cout<<"  "<<#X": "<<X<<endl;
#define FOR(i, m, n) for(int i(m); i < n; i++)
#define F(n) FOR(i, 0, n)
#define FF(n) FOR(j, 0, n)
#define aa first
#define bb second

typedef long long ll;
typedef vector<ll> vi;
typedef pair<ll, ll> ii;
typedef vector<ii> vii;

int dp[5123][5123], a[5123];
int n;

int dxh(int l, int r) {
    if (l >= r) return 0;
    if (dp[l][r] != -1) return dp[l][r];
    int bst = 0;
    if (a[l] == a[r]) bst = dxh(l+1, r-1) + 1;
    bst = max(dxh(l+1, r), bst);
    bst = max(dxh(l, r-1), bst);
    return dp[l][r] = bst;
}

int main() {
    while (scanf("%d", &n) == 1) {
        F(n) scanf("%d", &a[i]);
        F(n) FF(n) dp[i][j] = -1;
        printf("%d\n", dxh(0, n-1));
    }
}