#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=short;
using ull=unsigned long long;
using ld=double;
using vi=vector<int>;
using vvi=vector<vi>;
using pii=pair<int,int>;

#define MX 5007
int n;
int v[MX];
ll DP[MX][MX];
ll dp() {
  F(n-1) DP[i][i+1]=(v[i]==v[i+1]?1:0);
  FOR(l,2,n) {
//    dout<<"|"<<l<<endl;
    for(ll i=0; i+l<n; i++) {
//      dout<<i<<' '<<i+l<<" - "<<DP[i+1][i+l]<<endl;
      DP[i][i+l]=max(DP[i+1][i+l],DP[i][i+l-1]);
      if(v[i]==v[i+l]) DP[i][i+l]=max(DP[i][i+l],ll(DP[i+1][i+l-1]+1));
    }
  }
  return DP[0][n-1];
}

int main(){
  for(;;) {
    if(scanf("%d",&n)!=1)break;
    F(n) scanf("%d",&v[i]);
    cout << dp() <<endl;
  }
  return 0;
}