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

#define FOR(i,a,b) for(auto i=a;i<b;++i)
#define F(a) FOR(i,0l,a)
#define FF(a) FOR(j,0l,a)
#define aa first
#define bb second
#define PB push_back
#define pb push_back
#define MOD ((int)1e9+7)
#define INF (1<<30)
#define LINF (1L<<61)
#define EPS (1e-10)
#define EQ(a,b) (fabs(a-b)<=(fabs(a+b)*EPS))
#ifdef DEBUG
#define DEB puts("DEB!")
#define DPRINT(X) printf("#X: %d\n",X)
#define dout cout
#else
#define DEB do{}while(0)
#define DPRINT(X) do{}while(0)
#define dout 0 && cout
#endif
using ll=long long;
using ull=unsigned long long;
using ld=double;
using vi=vector<int>;
using vvi=vector<vi>;
using pii=pair<int,int>;

#define MX 5007
ll n;
ll v[MX];
ll DP[MX][MX];
ll dp(ll a, ll b) {
  if(a>=b) return 0;
  ll &ret=DP[a][b];
  if(~ret)return ret;
  ret=0;
  if(v[a]==v[b]) ret=1+dp(a+1,b-1);
  ret=max(ret,dp(a+1,b));
  ret=max(ret,dp(a,b-1));
  return ret;
}

int main(){
  ios::sync_with_stdio(false);cout.tie(0);cin.tie(0);
  for(;;) {
    cin>>n;
    if(!cin)break;
    F(n) cin>>v[i];
    memset(DP,-1,sizeof(DP));
    cout << dp(0,n-1) <<endl;
  }
  return 0;
}