#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<LL, LL> PII;

// chmax, chmin
template <class T>
inline bool chmax(T &a, T b)
{
  if (a < b)
  {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T &a, T b)
{
  if (a > b)
  {
    a = b;
    return 1;
  }
  return 0;
}

template <typename T>
function<T(T, T)> op_max = [](T a, T b) -> T { return max(a, b); };
template <typename T>
function<T(T, T)> op_min = [](T a, T b) -> T { return min(a, b); };

template <typename T>
function<T(T, T)> op_sum = [](T a, T b) -> T { return a + b; };

namespace detail
{
  template <typename T, std::size_t NDIMS>
  struct vector_builder
  {
    using type = std::vector<typename vector_builder<T, NDIMS - 1>::type>;

    static type make(std::vector<std::size_t> dims, const T &v = {})
    {
      const auto vec = std::vector<T>(dims.empty() ? 0 : dims.back(), v);
      if (!dims.empty())
        dims.pop_back();
      return vector_builder<std::vector<T>, NDIMS - 1>::make(dims, vec);
    }
  };

  template <typename T>
  struct vector_builder<T, 1>
  {
    using type = std::vector<T>;

    static type make(std::vector<std::size_t> dims, const T &v = {})
    {
      return type(dims.empty() ? 0 : dims.back(), v);
    }
  };
} // namespace detail

template <typename T, typename... SIZE_T>
auto exvector(const T &v, SIZE_T... dims)
{
  static_assert(sizeof...(dims) != 0, "invalid dimension");
  std::vector<std::size_t> vec_dims{dims...};
  return detail::vector_builder<T, sizeof...(dims)>::make(vec_dims, v);
}

using namespace std;
//********************  dumps ************************//

template <typename T>
void dump(const T &data, vector<int> &iter)
{
  cout << data << " ";
}
template <typename T>
void dump(const vector<T> &data, vector<int> &iter)
{
  for (auto elem : data)
    dump(elem, iter);
  cout << endl;
}

template <typename T>
void dump(const vector<vector<T>> &data, vector<int> &iter)
{
  for (auto elem : iter)
  {
    cout << "[" << elem << "]";
  }
  cout << endl;
  for (int i = 0; i < data.size(); i++)
  {
    iter.push_back(i);
    dump(data[i], iter);
    iter.pop_back();
  }
  cout << endl;
}

template <typename T>
void dump(const vector<T> &data, int dummy)
{
  for (int i = 0; i < data.size(); i++)
  {
    cout << "[" << i << "] " << data[i] << endl;
  }
}
template <typename T>
void dump(const T &data)
{ // T : data, U = base type val.
  vector<int> iter;
  dump(data, iter);
}
///////////////////////////////////////////////

void solve(long long N, std::vector<long long> A)
{
  LL INF = 1000000;
  INF *= INF * INF;
  vector<vector<LL>> dp(4, vector<LL>(N + 2, -INF));

  for (int i = 0; i < min(4LL, N); i++)
    dp[i][i] = A[i];

  LL ans = -INF;
  for (int i = 0; i < N; i++)
  {
    for (int j = 0; j < 4; j++)
    {
      for (int k = j; k < 4; k++)
      {
        int diff = k - j + 2;
        int next = i + diff;
        if (next >= N)
          continue;
        chmax(dp[k][next], dp[j][i] + A[next]);
      }
      if ((i - j) / 2 + 1 == N / 2)
        chmax(ans, dp[j][i]);
    }
  }
  cout << ans << endl;
}

int main()
{
  cout << setprecision(10);
  long long N;
  scanf("%lld", &N);
  std::vector<long long> A(N);
  for (int i = 0; i < N; i++)
  {
    scanf("%lld", &A[i]);
  }
  solve(N, std::move(A));
  return 0;
}


Measured memory_bytes: 12732