#include <bits/stdc++.h>

using namespace std;

int main()
{
  int num;
  cin >> num;

  vector<int> v(num + 10, 0);
  for (int i = 0; i < num; ++i)
  {
    int tmp;
    cin >> tmp;
    v[i] = tmp;
  }

  long long result_o = 0;
  long long result_x = 0;

  for (int i = 0; i < num; i += 3)
  {
    vector<int> vo(3, 0);
    vector<int> vx(3, 0);

    // oxo, xxo, xxo
    vo[0] = result_x + v[i] + v[i + 2];
    vo[1] = result_o + v[i + 2];
    vo[2] = result_x + v[i + 2];
    // xox, xox, oxx
    vx[0] = result_o + v[i + 1];
    vx[1] = result_x + v[i + 1];
    vx[2] = result_x + v[i];

    result_o = *std::max_element(vo.begin(), vo.end());
    result_x = *std::max_element(vx.begin(), vx.end());
  }

  cout << ((result_o > result_x) ? result_o : result_x) << endl;
}